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He liked to call it his Hal time. Between the wee hours of sunset and sunrise, the Alien Zoology lab at the University of Mocon City was peaceful and dark, the only sounds coming from the animals under his care.

Professor Hal Mon listened as they chittered and chattered away in their cages – creatures from over a dozen different worlds. There was so much to learn from them. At this hour as he fed and tended to each one he liked to talk to them, share a little story about his day, take each one out for a bit of attention. Every creature needed some attention now and again. Most species thrived on social contact, and the animals were the closest thing he had to family, well, apart from his colleague Professor Ben. But even Ben didn't enjoy spending as much time with the animals as Mon did. To Ben they were just his work.

"Hey now, my darling, and how are you this fine evening," Mon asked as he removed a small, furry specimen from its cage.

It was the rarest animal in his lab. Round, soft and shaped like a small loaf of bread, its fur resembled a bristly bottle brush of rust and black stripes. Its eight stumpy limbs were so small as to be almost nonexistent, and in fact it looked and moved quite like a very large caterpillar. This might have been off putting but for the fact that it made the most inviting little mewling noises, like a kitten, and indeed it purred when it was held in just the right way.

Professor Mon was busy feeding it small pellets from his hand when he heard a noise coming from the front room. It was the unmistakeable click sound of the door latch to the lab. He clutched the animal a bit tighter and crept forward. The cleaning crew had already gone through and his colleague Ben never came this time of night. Mon knew there were reports of thieves in the city, pillaging hospitals and universities for items to sell on the black market. The creature he held had been hunted to near extinction on other worlds and he would lay down his life to protect this one. Very carefully, he peered from a crack in the door to the outer area of the lab.

They appeared to be droids, two of them, identical, dressed entirely in black and vaguely humanoid in appearance but with luminescent skin and emotionless, unblinking eyes. Having entered the lab, they were now quietly making their way towards the back, where Mon hid. One of them held a plasma weapon and that was what convinced the Professor they were not here for a friendly chat.

Mon looked back at the other animals sadly. He only had two hands, which at the moment were full of a fuzzy endangered creature that was drifting off to sleep. It was all he could do, though it pained him that he could not attempt to save them all. They did keep a weapon for protection in the lab, but it was in Ben's desk drawer out front, where the droids were now. He needed to get out. If he escaped he could call for the police.

As quietly but as quickly as he could, Professor Mon placed the sleeping creature in the large pocket of his lab coat, lifted the window and climbed out into the night.

~*~

"Mocon City!" The Doctor announced triumphantly as he and Donna emerged from the TARDIS. "Home of the Great Intergalactic Music Festival!"

"Ooh I like festivals," Donna said, "so much to see."

Leaving the TARDIS parked in a small side street, the Doctor locked the door and they walked down to the corner, where they soon came upon the well lit high street. They turned in the direction of the crowds and strolled, mixing in. Most of the people were humanoid like the Moconians, but this being an intergalactic festival, there were tourists of the non-humanoid variety as well, peppered in like seasoning in a stew, taking in the evening's diversions. Although it was late at night, the streets were filled with people of every height, width and colour imaginable.

"This planet is a hub," the Doctor explained, "perfect central location. Visitors come here from across the galaxy, scores of worlds converging to share something they all have in common, a love of music. It's a universal language, Donna. Performances take place each day, and at night its one big party. Brilliant, isn't it?"

The Doctor glanced at Donna, thrilled by her expression as she took it all in. The shops and pubs were all open, and sounds of conversation and music drifted out into the street from each establishment creating a cacophonic cocktail as interesting as the sights themselves. Street performers - musicians, artists, magicians and acrobats - were all about and the atmosphere had the buzz and joy of New Year's Eve.

They bought some tall lime coloured drinks from a street vendor and watched as a painted bald man performed an act of hands free pin juggling by means of telekinesis.

"I'd like to see them try that back home," said Donna, as they applauded and she tossed a credit chip into the man's upturned hat.

They walked on, the Doctor taking long strides, hands deep in his pockets, already looking for something else to feed his short attention span as Donna hurried to keep up.

"I wouldn't mind doing some shopping," Donna called out after him, "maybe bring back something for my granddad."

The Doctor stopped and looked around for a suitable establishment to suit her request. He was just about to suggest one when he heard a woman scream from nearby.

"Help! Somebody stop him! He's got my purse!" she cried.

He spun round. A small, elderly lady was being mugged by a purse snatcher across the street. They had walked so long they had reached an area where the crowd had thinned, though it appeared that the few people closest were simply watching, stunned, but not reacting. The thief now had the purse and had taken off around the corner, leaving the frightened victim standing, apparently unharmed.

"Oy!" called the Doctor, running off after him.

From behind him he heard Donna, also giving chase. "I'll go down the next block and maybe we can corner him," she said as she ran.

They split up and the Doctor turned down the nearest alley where the man had gone. The way was long and narrow, the light dimmer here but he could just make out a moving shape at the far end. A chain link fence blocked egress. Suddenly he saw a purse shaped object fly over the top of the fence and the man clambering up after it. The Doctor was only halfway down the alley now, and he knew he'd never reach him before he went over, but then he spotted something. A second smaller object fell straight downwards on his side of the fence. It was all in shadow and the Doctor couldn't tell what it was. Thinking it might be another stolen wallet, he stopped to retrieve it, giving up his pursuit of the man who had by now dropped down on the other side and was long gone.

The Doctor hunched over for a moment to catch his breath, his eyes scanning the alley pavement around his feet at the same time. There was nothing in plain sight so he looked amongst the rubbish bins and boxes that were piled along the fence. Curled up into a little quivering ball was a small animal. Not at all comfortable with its surroundings like rats and cats would be in an alley on Earth, the Doctor could tell it was lost and afraid. It made a tiny sound like it was crying for help.

"Oh hello," said the Doctor, slowly reaching down with both hands to pick it up, "did someone purse snatch you? It's all right, I've got you now."

The creature was trembling harder and the Doctor brought it closer and let it nestle in his coat. Striped and bristly, with a pug face almost completely hidden by its fur, it looked like nothing he had ever seen. He was anxious to show Donna and then perhaps find its owner.

"Let's get a better look at you," he said.

Bringing his hand around to the creature's face, he lifted it up to examine it in the dim light, checking for a collar or identification marking.

And then the animal lashed out with a small shriek and bit him.

"Ow!" the Doctor gasped.

He was so surprised that he pulled his hands away and dropped it. He watched as the animal landed safely to the floor and began to scurry away quickly, back behind the bins.

"You didn't have to do that," said the Doctor, clutching his left hand.

Droplets of blood were rising from deep puncture wounds just below his wrist caused by pointed teeth. It didn't look like much but his hand had already begun to pulse and throb and he felt lightheaded. He swayed in place as he tried to collect his thoughts.

What was it he had meant to do? Show Donna, that was it. Show Donna what, he wondered to himself.

He wasn't sure.

"Wait..." he murmured as his vision grew fuzzy, reaching out towards the rubbish pile.

He shook his head and blinked to try and clear it.

"Where did...errrr...Don..."

And then the pavement threw itself out like a rug from under his feet. The alley spun in a circle and turned completely black.

~*~

Donna had run around the next corner hoping to catch the man at the back of the alley, but her path led to a dead end at a brick wall. Taking the next street over, she reached the end of that one and found another empty, narrow street that wound into several more, with no sign of her quarry in sight. She had no idea how far she'd gone so before she could get too disoriented and lose her way, Donna turned around with a sigh and walked back to find the Doctor.

He had gone down the first alley, nearest to where the woman had been mugged. Donna turned back onto the high street. The police had already turned up and were taking statements from witnesses. She didn't see the Doctor there, so she turned down the corner where he had chased the thief.

Halfway down the long alley she heard a groan through the darkness and saw the shape of something large moving on the ground.

"Doctor?" she called, jogging the rest of the way.

The Doctor was now hunched over, pushing up to his hands and knees, moving slowly.

"Yeah," he answered thickly. "I'm fine."

Donna knew he would say that even if his head were half off his shoulders. She tried to get a better look at him but the light in the alley was too dim.

"What happened," she asked, taking his arms and helping him up to stand, "Did he hit you?"

The Doctor ran a hand through his hair. "What? No... no... I don't think so."

She wanted to ask what he had been doing on the ground then, but knowing the Doctor he was probably licking the pavement to track the thief's scent or some other barmy thing. She turned and saw the chain link fence.

"So he got away then?" she asked.

"Who?" asked the Doctor, looking slightly confused, scanning the alley.

"The man we were chasing," Donna said, "I ran around back a few streets over but I never saw him come through. Did you see which way he went?"

The Doctor didn't seem to have heard her. He was turning slowly on the spot, taking in his surroundings, looking up and down, as though he didn't quite know how he had gotten there. Donna followed his gaze but saw nothing but a dark alley and rubbish bins.

"Are you sure you're all right?" Donna asked him.

"Yes, of course I'm sure," the Doctor said quickly, turning back to her, "I suppose he got away, then... this...person."

Donna shrugged, "Well, we tried. At least we can give a description to the police, they're here now."

Donna led the way back to the high street and the Doctor followed. A block away they joined the crowd who were giving their statements to the officer on duty. When it was her turn Donna spoke.

"We ran after him but he got away," she said, speaking into the officer's handheld recorder. "He had a blue jacket, a bit worn, and dark trousers...thin light hair..." She tried to estimate his build with her hands. "Oh I dunno, I only saw him from a distance but he wasn't very tall was he?" she asked, turning to the Doctor for confirmation.

The Doctor looked back at her blankly for a second. Finally he nodded, "Yes. That er...sounds about right. Seems you got a better look than I did. It was dark."

They finished with the police and went on their way, back in the direction of the festivities, both of them moving a bit slower. For her part, Donna felt a bit deflated. She watched the street performers from her place on the sidewalk as they walked by but all the joy had gone out of it for her. The Doctor didn't seem to be enjoying himself either. Donna thought he looked a bit queasy, if Time Lords ever got that way.

"You know what, I think I've lost my taste for the music festival," said Donna, "I've seen about enough of this world. Let's go somewhere else. A fresh start, whatcha think?"

"I think that's a good idea," agreed the Doctor, frowning.

~*~

The man hid behind some bushes in the park while he caught his breath. He shivered, he was freezing. Next time he would have to steal a heavier coat. He searched through the purse. There were fifty credits in the wallet. That would be enough to provide meals for himself and his mate for the next three days, if he was careful. Three days before he would have to steal again, not because he wanted to but because he didn't know what else to do. He pocketed the chips and stashed the rest under the shrubs.

Now he had to go back and find his little friend.

He had only realised he had dropped him when it was too late to go back. That tall man in the long brown coat had chased him all the way to the fence and he couldn't risk being stopped. He was alone in this place, and he had committed too many crimes to show his face anywhere. But his friend was scared and lost like he was, and he had promised to look after him, so he would go back now. Hopefully the man had given up and moved on.

Retracing his steps, he reached the fence and climbed back over it. The alley was quiet and empty again, the sounds from the evening's merriment now dying down in the distance in the wee hours. He knew he liked this time of night, when all was peaceful and still, but he couldn't recall why.

He got down on his knees and made little cooing noises that the creature responded to. He didn't know the animal's name any more than he could remember his own. He didn't even know its genus or where it came from. All he knew was that they seemed to need each other now, and that together they would survive.

"Where are you now, my darling," he whispered. "I'm here, you can come out."

A little flat face poked out from behind the rubbish bins, shaking a bit but otherwise unharmed. It mewled. The man smiled, relieved, and carefully scooped it up.

"I'm sorry for leaving you behind, but I'm back. I promised I'd take care of you," he said to it, petting its fur gently.

Clutching his friend, the two went out into the night.

~*~

"Doctor, where are we going?" asked Donna after fifteen minutes of walking ever darkening streets.

His head was still swimming but at least he knew that. "To the TARDIS," he explained, as though it should be obvious.

"But why are we going this way, I was sure we left it back there," said Donna, pointing to the row of now closed cafes in the opposite direction, "I remember those tables and chairs. We had to step around them when we came out the side street."

The Doctor stopped and scratched his head. "Did we? That's odd. I usually have an excellent sense of direction."

"Well, never mind that. Come on, it's this way, I'm sure of it," said Donna, taking the lead.

He didn't admit it to her but the Doctor felt actual relief at seeing his ship again. This was all too weird and the sooner he was off this planet the better. He fumbled around in his pockets for his key, finding it in the third one he checked, unlocked the doors and they stepped inside.

"Now then," the Doctor started, tossing his coat over the usual Y shaped coral formation and feeling perkier already in his familiar environment, "where to next? I know this one planet that's 99% water. The roads are canals and everyone swims to get around, it's called... uh...erm..."

"That's a funny name for a planet," joked Donna.

The Doctor snapped his fingers, "It's called...aw blimey, I can't remember what it's called. Well, how about somewhere in the past then, we can go back to ancient Egypt and watch them build the pyramids! I owe Cleo a visit, it's been a while."

"Oh that sounds lovely," said Donna.

"There we are then, Egypt, B.C.!" Smiling, the Doctor went to the console, reached out and then stopped cold at the sight of all the dials and buttons.

So many dials, buttons, levers and knobs. Were there always this many? And there was something more, he was sure of it...

To visit a point in the past he needed space time coordinates. What were they? He knew these things as easily as breathing. It was all there in his head, the coordinates for every known corner of the universe in both space and time, stored in his vast Time Lord brain. Now the TARDIS console looked as strange and unfamiliar as...well, as it would look to a human.

He felt his hearts begin to race one another as he realised for the first time that something was terribly, terribly wrong.

"Doctor, what is it?" asked Donna, stepping up close to him, no doubt concerned over his near frozen state.

"Tell you what," the Doctor told her, head snapping up, trying to distract her before she noticed him breaking out into a sweat, "Let's just pick a location at random and see where we end up. It'll be like a game."

"No," said Donna, eyes wide, looking as frightened as the Doctor felt, "Tell me the truth. You couldn't find the TARDIS before, you forgot the name of that water planet, and that man we were chasing..."

She paused, putting it all together, and then gasped, touching the Doctor's arm, "You really don't remember him do you? You said you didn't get a good look at him, but when I first asked you, you didn't know who I was talking about. And now you've forgotten how to fly the TARDIS! Don't deny it. Doctor, something's happened to you, what is it?"

"Donna," he started, with a bit of effort to sound calmer than he felt, "you're overreacting, I'm sure it's nothing..."

Donna shook her head, "I don't think so. You have an extraordinary memory, this isn't like you. We have to go back, find out what happened."

The Doctor sighed and sank down into the seat behind him. He couldn't keep anything from Donna, she always knew. He didn't want to worry her, but he didn't like this sensation one bit. He'd had amnesia before after a past regeneration that went wrong as they often did. It wasn't a pleasant experience. Since the Time War he had very little left to call his own, his memories were just about the only things. If he lost those then what would he have? And what was he likely to forget next – Donna? Gallifrey?

...Rose?

"Doctor?" asked Donna gently, concern in her voice as it appeared she had been trying to get his attention for some time.

He had drifted off, miles away. He looked up at her, all trace of bravado and bluster gone. "It's mostly just recall for now - names, places...random, unrelated things, but I think it's getting worse."

"What do you remember about the purse snatcher and the alley?" she asked him. "Think. Did he have a weapon, did he speak to you, did you fall..."

The Doctor closed his eyes and tried to think. "Errr, I was running...I don't know why. Then I woke up and heard your voice. That's all. I can't remember anything in between."

"So you were unconscious?" Donna surmised.

"I must have been, but why? And what does that have to do with this?" the Doctor asked her.

"Let's go back to that alley, maybe we can find out," she said.

~*~

By the time they returned to the alley it was dawn, the sun casting a fiery glow over the streets, providing a bit more light than they had previously. Donna and the Doctor did a visual sweep of the area, checking behind the bins and rubbish but there was nothing there.

The Doctor had his hand out, trying to pick up on any unusual energy signatures that shouldn't have been there. After several minutes he dropped his hand in frustration and rubbed his eyes.

"Nothing," he said. "I don't understand it. There's nothing here."

"Maybe it's something more subtle," suggested Donna. "How about your sonic screwdriver, would that help?"

The Doctor looked at her like she had suddenly started speaking in tongues.

"My what?" he asked.

Donna sighed, came closer and began digging through the Doctor's coat and suit pockets. He looked down, surprised.

"What are you doing?" he asked.

"Just hold still, Spaceman," she said and removed the small wand like object from his inside jacket pocket. She held it up to show him. "Ring any bells?"

He took the device and turned it round in his hands. Pointing the business end at his own face he pressed a button and the light combined with the sonic wave made him blink and jump back, startled. He handed it back to her carefully between two fingers as though it might explode.

"I don't know what to do with that," he said, "I'm sorry."

Donna took the screwdriver back from him with a sympathetic smile. "Don't be sorry, it's not your fault. But I'm afraid we might be running out of time. We need to find that purse snatcher, I'm sure of it, that's when this all began. He must have done something. Let's see if the police have found him."

A visit to the police station turned up nothing. They walked out and back onto the streets in despair. They only wanted to leave this world and now it seemed they were stuck there until they could solve this mystery and cure the Doctor, but they didn't have a single lead, just a missing purse snatcher who could be anywhere by now.

It was mid-morning, and all over the city the music festival was starting in halls, squares and outdoor pavilions. Donna had almost forgotten that was the reason they had come to Mocon. She wondered whether the Doctor remembered it at all, or was it fading away to nothingness along with everything else.

They walked some more, neither of them saying very much. Donna scanned the flyers for the festival performances in all the shop windows, thinking of how different their trip might have been. Then she stopped when her eyes locked on one particular poster.

It was a notice for a missing person, and she recognised the face of a pleasant, smiling, middle-aged man. It was their purse snatcher.

"Doctor!" she said as she stopped and pointed. "That's him."

It stated that his name was Professor Hal Mon and he had worked at the local university in the zoology department until his disappearance a week ago. Donna felt like cheering. The man may still be missing, but they had a name and an employer. They were one step closer to finding help.

~*~

Professor Mon was listed as a member of the Zoology Department at the University of Mocon City specialising in alien species. He had a campus laboratory where he did his research along with his colleague Professor Dav Ben. Donna and the Doctor arrived at the lab. It was an ordinary research facility by all accounts, no different from that on any other world, with a front room of equipment, models and tables and a smaller room in the back.

"Excuse me," said Donna, upon entering and finding a white haired man in a lab coat, "Are you Professor Ben?"

The man removed a pair of half glasses and looked up from his test tubes.

"Yes, can I help you?" he said.

They stepped fully inside and Donna continued, "My name's Donna Noble and this is the Doctor. We're looking for Professor Mon. We were hoping you might be able to help us find him."

The Professor stiffened, "The police are already on the case. As far as I know he is still missing. Who are you? Let me see some identification."

Donna knew this was typically the time when the Doctor would flash his psychic paper that would convince the reader they were whoever the paper said they were. Given his current condition however, she wasn't surprised to find the Doctor wandering the lab instead, completely apart from the goings on. Donna glanced his way at first for assistance out of habit, but the Doctor was looking around the lab absently, muttering to himself.

"We're private investigators," she explained simply, turning back to the Professor. "My partner has our credentials if you could just hang on a minute."

Donna crossed over to where the Doctor stood. He had his back to the room, appearing to examine a model of what may have been the Moconian solar system on a side table, but when she got close she heard him reciting with intense concentration. It was something she had first learned in school.

"Hydrogen, Helium, Lithium, Beryllium, Boron... Carbon...uuuuugh come on, come on...," he growled as he slapped himself in the head repeatedly.

"Nitrogen, I think," said Donna quietly.

The Doctor turned to look at her, surprised to find her standing there and equally surprised to hear her response. She imagined it was like the Time Lord equivalent of learning the alphabet to him.

"I temped for a pharmaceutical company once," she explained, "They had one of those periodic tables framed up in reception and I would stare it all day while I answered phones. Boring job, reception."

Donna put a hand on his shoulder and the Doctor smiled.

"Excuse me for a minute," she said as she reached into his pockets again. This time she produced a small wallet. She opened it and showed it to the Professor, who looked at it, nodded and returned it.

"It's important that we find him," Donna told the Professor, returning the psychic paper to the Doctor, who had turned his attention back to the reason they had come and stood beside her, "He has information that we need."

"I'm sorry I can't be of more help," said Professor Ben. "We had a break-in, and the next day he failed to appear at work. I haven't seen him since."

"Was anything stolen?" asked the Doctor.

"A few rare animals," he replied. "Professor Mon and I study alien zoology. Some of our specimens are highly prized and quite often the targets of thieves."

The Professor indicated towards the back of the lab. They followed him into another room and found a wall of cages containing various small animals. Donna looked. The only one she recognised was an Earth rabbit. The rest came from different worlds. Several of the cubicles were empty.

She watched the creatures for a bit, and the feeling of despair slowly returned. They were no closer to an answer, and all the while she could sense the Doctor slipping away. Standing beside him, she could tell how worried he was, even as he tried his best not to show it to her.

The Doctor was studying the animals too, taking more notice of the empty cages than she had, no doubt wondering what had once been inside them and whether they were all right. As he pondered, Donna watched him. He was idly scratching at his left hand where for the first time she noticed a small wound.

"What's that there?" she asked him, taking his hand.

The Doctor looked down at the marks just below his wrist and the reddened area around it. "I don't know," he said, "I just noticed it."

"It looks like a bite," said Donna.

She turned to Professor Ben who had been in the room with them. "Have you seen anything like this Professor?" she started to ask him, but he was no longer there.

He had gone some time earlier, Donna hadn't noticed. Now he was back, standing in the doorway to the front room holding a gun on them. They both looked up surprised, and the Doctor took a small step in front of Donna, hands raised.

"I knew you were lying," said Ben, "I noticed that bite wound when you first approached the cages. What did you do with Mon? Is he dead? Where are the creatures?"

"Professor, we didn't kill anyone, honestly," said the Doctor.

"We saw Professor Mon yesterday, snatching a woman's purse in the High Street," explained Donna. "We ran after him but he got away."

"Then where did you get that?" Ben asked, pointing to the Doctor's hand with the gun.

"I can't remember," insisted the Doctor. "That's why we're here. I promise you, we're here to help."

The Professor seemed to contemplate the Doctor's words, and they somehow had a calming effect. He lowered the gun.

"I'm sorry," Ben said. "I thought you were working for the thieves. It's the third time in eight months that animals have been stolen. They turn up later in markets if they're ever found at all, sold off bit by bit for their valuable parts. It's all been very stressful."

"It's all right, we understand" said the Doctor. "I'm sorry about your animals. We'll try to get them back, but what can you tell us about this bite?"

"You were bitten by a Dolian chansatt, a rare endangered creature," the Professor explained, pointing to one of the empty cages. "It was stolen from there. It releases a powerful toxin attacking the engrams that encode memory and breaking them down. When it bites, its attacker quickly forgets about it and moves on."

"Good defence mechanism," said the Doctor, "I can tell you it works."

The Professor continued, "When Mon and the animals disappeared, I suspected foul play. The chansatt is our most valuable creature, prized for its venom, which is both used and misused in small doses as an amnesia drug. It can fetch large amounts of money on the black market. The animals are hunted down in the wild, there have been thefts from labs and zoos on other worlds, and we had the only known chansatt in captivity on this planet."

"But Mon hasn't been kidnapped, we saw him," said Donna. "And why would a university professor go about nicking purses?"

"Because he doesn't know he's a university professor," the Doctor replied, realising, "He's been living on the streets and stealing to eat because he's forgotten where he lives and works. Professor, was Mon bit by this creature?"

"It appears as though he must have been," said Ben, "possibly in trying to escape with it, to protect it from the thieves. The chansatt is mostly docile, but I would imagine it was frightened."

"If you were bit as well Doctor than you must have found them both together," said Donna, as the pieces quickly fell into place.

"And If Mon is a victim too, wandering the streets with the chansatt they are both in danger, and vulnerable to the criminals who may still be after them," noted Ben.

The Doctor turned to him, "Professor, the venom has already begun to take effect on me. There are huge gaps in my memory and they're getting wider. Will it wear off in time or is there an antidote required?"

"I'm sorry, but the effect is progressive and irreversible without an antidote," said Ben, "We were working on developing one as part of our study, but to complete it we require the chansatt and a sample of its venom."

"Then we're back where we started," said Donna. "We need to find the Professor and the chansatt."

"We'll do everything we can to find them," the Doctor told Ben, "and then we'll make the antidote that will help us both."

The Professor nodded, "I would like to accompany you if I may. I know my way around the city, I know what the chansatt looks like and how it behaves, and Mon must be very frightened. If he sees me something may trigger his memory, get him to trust me."

They agreed and set out to leave the lab together, the Professor leading the way. As they walked, Donna stepped up quickly to catch up to Ben.

She spoke softly, leaving the Doctor several steps behind, "You said this is the only chansatt on the planet. What will happen if we can't make an antidote?"

"Victims of chansatt poisoning eventually go mad," said Ben, "That is the third and final stage of the disease. The rate of progression differs according to physiology but after a week, Mon would be in that stage. He likely no longer knows who or where he is. I can't be sure about your friend."

Donna's stomach did a flip, her thoughts focused on the Doctor and his grim outlook. Would the poison progress faster for a Time Lord than a Moconian, fed and nurtured by the Doctor's large memory capacity, giving the toxin plenty to grow on? In less than 24 hours he had already gone from forgetting the creature's bite to losing staggering amounts of information. Donna didn't know exactly how much time they had, but she knew it couldn't be long.

"Now we just have to hope that we find the chansatt before the criminals do," she said.

"So that should give us all the incentive in the world, don't you think?" said the Doctor loudly from behind, having heard every word they said.

Donna looked back and saw the Doctor's devilish grin.

~*~

Wharf district search complete. Subject is not present. Designate new location.

Droid One relayed its message to Droid Two on the communication frequency they shared. Since the subject's disappearance from the laboratory they have searched continuously, combing nearly every square centimetre of the city. They needed neither food nor sleep, so the search would continue until the mission was complete. The creature would be located and sent off world where it would fetch millions. There were no alternative orders.

The droids were programmed with the subject's scent for tracking, and they knew they came close several times, but each time the creature had somehow eluded them. It was being protected by the Moconian.

The droids had only limited reasoning skills, necessary to perform their functions. They were not programmed to use judgment, mercy or compassion. They were programmed to retrieve the subject and kill whoever got in their way. The subject would be acquired. It was as simple as that.

Destination Central Transport Depot. Continue search.

Affirmative.

Leaving the docks, they turned as one in the direction of the transport depot, where the many homeless of the city spent their nights, and those on the run may seek to escape.

~*~

Led by Professor Ben, Donna and the Doctor searched the streets of the city at twilight, under the bridges and places where the vagrants congregated. Carrying one of the notices, they showed Professor Mon's photo around and asked everyone they could find if they'd seen him. All they received for their trouble were blank stares, shaking heads and open hands, begging for scraps.

It was overwhelming, the sheer volume of the displaced. Donna wished she had something to give them all, but there was nothing she could do and for the Doctor's sake she knew she had to stay focused on their task. Even as they searched Donna could tell that the Doctor was growing more scattered and confused.

"Donna, why are we here?" The Doctor asked her after thirty minutes scouring the riverbank.

"We're looking for someone," she explained patiently, "Someone who's lost."

Someone like you, she thought, but she kept herself from saying it. If he no longer understood enough to be worried she didn't want to complicate matters.

Ten minutes later he asked her again.

Their search came up empty at the riverbank, under the motorway and at all of the area shelters. Professor Ben suggested they try the transport depot in case Mon was attempting to leave the city. It was also a common place for shelter, he said.

Like a curious but persistent young child the Doctor's questions continued as they made their way to the depot - What were they looking for, where were they going, who was lost. Donna quickened her pace, fighting back tears, not stopping until they arrived at the depot. Mon simply had to be here, that animal was their only hope.

"Donna," she heard the Doctor say again from behind her when they entered the station.

"What?" she replied over her shoulder, glancing this way and that for the lost Professor.

"Donna," he repeated.

Donna sighed. "I said wha...," she started to say again, but then stopped when she turned around.

The Doctor hadn't been asking her a question. He was standing there, repeating, like you do when you're trying to memorise something.

"Donna," he was saying to himself, looking at her, a fistful of hair in his hand, "Donna, you're Donna, you're called Donna..."

She had been trying so hard not to cry, but at this her heart broke. She came to him, carefully took his hand from his head and held it, and a tear escaped and ran down her cheek. It was something about his eyes. He looked at her like he was falling from some great height with nothing to grab on to.

"That's right, I'm Donna. You just stay with me Doctor and don't go wandering off," Donna told him.

Despite the tragedy of the situation she smiled and laughed softly at her own words.

"What's so funny?" the Doctor asked her.

"Nothing. It's just that you're usually the one to tell me that, not to go wandering off. Do you remember?" she said.

The Doctor just stared, more troubled than she'd ever seen him.

"We'll find them Doctor," she said.

Their search of the station yielded nothing, so they followed Professor Ben out past the tracks where the vehicles were stored. More vagrants sat scattered here and there like worn out parts in a scrap yard, watching these odd, well dressed intruders show their photo around. This time they were clever and brought sandwiches from the depot lunch counter which they passed out along with their questions. The crowd were much more receptive, though most still had no useful information.

Donna handed her last sandwich to a young man sitting by a small campfire. With plunging expectations, she half heartedly showed him Mon's photo.

"I don't suppose you've seen this man about have you?" she asked him with a shrug.

The man took the sandwich and the notice and squinted, examining it carefully in the firelight's glow. "Yeah, yeah I sure have."

"You what?" she asked, shocked.

The man smiled, "I said I've seen him. He's been spending most nights here. He's a bit mad, so most folks here just leave him alone. He's got this funny looking pet you see and he's real protective of him. Anyone gets too close and he runs away."

~*~

"Professor Ben!" yelled Donna, running with the Doctor in tow. "He's here! Mon's here. He's been sleeping down that way, under the bridge, come on."

She led them down past where the depot ended and the tracks crossed one another via a multi-level bridge system. The man who had recognised Mon's photo had said that was where Mon slept. They were all running now, the bridge just coming into view, when they heard a sharp terrified scream rip through the silent night.

Donna could make out two dark shapes standing over a third shape on the ground. The one on the ground was cowering in fear, cradling something tightly. As Donna got closer she could tell that the standing figures weren't humanoid.

They were robots.

One of them held a weapon on Mon, who Donna could now see trembling on a pile of dirty blankets with the chansatt in his arms, squirming to be free.

"No, he's my friend, you can't have him! Please! Who are you?" Mon pleaded.

In response the robot pointed the weapon and fired.

"No! Stop!" cried the Doctor.

Mon's chest exploded with the impact of the plasma gun.

"Mon!" yelled Professor Ben, running and dropping at his friend's side and bursting into tears.

For a moment Donna thought the Doctor was his old self. Furious at the senseless killing, he turned on the armed robot.

"Why did you do that?" demanded the Doctor.

The robot said nothing, but just trained his weapon on the Doctor who stood his ground and stared it down. Donna's gaze shifted between the contrasting sights of an indignant Doctor at gunpoint and a distraught Professor, and then she remembered they had been looking for the chansatt. The second robot hadn't forgotten, and was already searching the area for it. Donna looked too, but found nothing. The animal had escaped from Mon's grasp just before he was killed.

The second robot returned and communicated something to the first with some high pitched sound that hurt Donna's ears. Then the first robot lowered his weapon and the two walked off.

The instant they were gone the Doctor and Donna joined Ben by Mon's body. The Professor was devastated. Donna didn't know how much the Doctor remembered about Mon but she knew he recognised the sight of someone in pain when he saw it, so it really didn't matter.

"I'm sorry," the Doctor said with a hand on Ben's shoulder.

"Those robots, they've gone after the chansatt." said Donna, "Mon was holding it. Did anyone see where it went?"

"Yes, I...," started the Doctor, standing up about to point, but then he stopped and spun around. "No. I thought I did, but...now I've lost it. It's like when you wake up from a dream. I was so sure I knew and then it faded away."

"I know," says Donna, "it's all right. I didn't see it either. I was too busy watching the robot with the gun. So what now? Should we follow them? Let them lead us to it?"

"But the droids will likely get to it first, there has to be a better way," said the Doctor. He paced in a tight circle and then with both hands slapped himself on the head again, "Arrrr, I wish I could think!"

"You could perhaps lure the chansatt to you and away from the droids," said Ben suddenly, his voice trembling. "Back at the lab, I was working on such a thing after Mon disappeared, a device to call out a mating signal at a sonic frequency, but I abandoned it when I was unable to find the proper wavelength. Perhaps you can succeed where I failed."

"Why would you think that we..." started Donna.

"You are not private investigators," said Ben, turning to face them both, his eyes red with tears. "The way you looked at my lab, Doctor, I know a scientist when I see one. You can fix it."

"In a fit state I'm sure I could but right now...," said the Doctor.

"We can try," Donna said.

"You should return to the lab," said Ben, "I will see to Mon first and then join you."

Donna knew the Professor wanted to contact the authorities and make sure than his friend received a proper burial. She was sorry they had been unable to help him in time, but at least his body would be identified. She and the Doctor made their farewells and walked back to the lab, with no sighting of the chansatt or the things the Professor and Doctor had called droids on the way.

"Who was that man, the one who was killed?" the Doctor asked Donna as they walked.

Donna tried to explain as simply as she could. "He was in trouble and we were trying to help him. He had this... sort of pet that's also in trouble and if we're lucky, we can save it so the man didn't die for nothing."

That seemed to be enough to suit the Doctor. Donna knew he would do anything he could, but by the time they reached the lab, the Doctor was worse than ever.

"Why are we here?" he asked her as they entered.

"There's a device here that will call the chansatt to us," Donna said. "We need to find it and fix it. And you may not remember why but I reckon you've still got that Time Lord brain of yours and can still work out how to do it." She described the device to the Doctor as Ben had described it to them earlier. "So look around and tell me if you see anything that could do that."

They searched shelves and drawers, cupboards and storage cabinets. They found all sorts of electronics but each time the Doctor looked and just shook his head.

"Now, this looks familiar," said the Doctor, holding up a small white box with a glowing blue light. He pressed some buttons, heard it whirr and smiled like a child at Christmas. "Oh that's clever!"

"Is that it?" asked Donna.

"Is that what?" the Doctor asked, still playing with the device.

"What we're looking for," she said.

The Doctor turned to look at her, confused. "Were we looking for something? I just liked it. It's a sonic generator. Well, almost. It's not working, but I've seen something like it before, I know I have..."

"Sonic, like your sonic screwdriver?" asked Donna.

"My what?" asked the Doctor.

"In your pocket," Donna said.

The Doctor searched his pockets and removed the screwdriver, holding it up. "That's where I've seen it! Brilliant!"

"Doctor," said Donna, as patiently as she could, pointing to the white box, "we need that device to send a call that the chansatt will hear. Can you fix it? And quickly?"

"Well I could, of course I could, but it would probably be quicker to use this since it's working already," he said, waggling the screwdriver between his fingers. "All I need is a sound file for the...what did you call it?"

"Dolian Chansatt," said Donna, "maybe on the computer?"

The Doctor went to the terminal Donna had indicated and typed in the name, searching for sound files of the creature's calls. When the results came up he squinted and pressed his nose to the screen. Donna reached into his pocket and produced his glasses, holding them out to him.

"Oh, that's handy, thanks," he said, taking them and putting them on.

He scrolled through the files, playing some samples, reading the descriptions.

"Ah, here they are," he said after a moment. "Well, that just involves a bit of voice recognition from the sound file converted into a sonic frequency. Simple really."

It took perhaps a few minutes more than it normally would have done because the Doctor had to experiment with the settings on the sonic screwdriver as though it had belonged to someone else. But as Donna suspected, he was as clever as always. When he was finished with the adjustment he held it up and pressed the button.

Ten minutes later, the chansatt came waddling into the room.

"Hello!" the Doctor said, delighted.

He went to pick it up and Donna stopped him. "Wait," she said, grabbing a pair of leather gloves from the table and tossing them to him, "use these, it bites."

"Really?" said the Doctor as he caught the gloves and put them on. "Hard to believe, it's so cute."

The Doctor carefully picked up the chansatt, but just as he did, the droids appeared in the doorway. One grabbed Donna and pointed its weapon at her head.

"Release the creature to us or the female dies," said the droid in a flat, mechanical voice.

"What, why?" asked the Doctor, petting the animal, "What's he ever done to you?"

"Doctor," Donna urged, terrified.

"All right, just let her go and you can have it," said the Doctor.

Donna hoped the Doctor had a plan beyond just rescuing her but in his state she doubted it. She also doubted he had any idea why that animal was so important to hold on to. The Doctor stepped forward slowly and handed the chansatt over to the other droid. Her heart sank as she realised that in giving over the chansatt the Doctor was sacrificing his own life for hers. There would be no hope of restoring his memory without that creature.

She wanted to protest and was about to tell the Doctor not to do it when several things happened at once. As the Doctor stepped back Donna heard the sound of a weapon discharging and she screamed. She felt something fly out and hit her in the back of the head and she thought she had been shot. She fell to the floor, burying her head in her arms.

After a moment when she realised she wasn't bleeding and was still alive she chanced a look around.

Behind her she saw Professor Ben, standing in the doorway with a smoking gun. The droid he had shot had released Donna, dropped its own weapon and fell when its head exploded.

The other droid still had the chansatt, but not for long.

Donna stood up, carefully assessing the bump on the back of her head from the flying bits of metal. The droid was handling the animal roughly in its hands, when the cute creature suddenly turned vicious. The chansatt shook, tensed and lashed out with a hiss, sinking its teeth into the droid's hand. Sparks flew and the droid shuddered as though it had placed its finger in an electric socket.

"Memory cascade failure!" it said, with more emotion than Donna would have previously thought it capable.

The droid sputtered, popped and collapsed in a smoking heap, joining its brother on the floor.

"Short circuit and works on computer memory too," said Donna, surprised. "Well done chansatt."

The little creature had landed next to the fallen droid and was now scurrying in a circle playfully, mewling away. Professor Ben took his jacket and quickly covered the creature and lifted it up.

"Are you unharmed?" Ben asked Donna.

"I'm fine. Now that we've got the chansatt we can make the antidote, right?" Donna asked Ben, anxiously, "We can help the Doctor."

As Ben nodded his assent Donna smiled and turned to look around the lab. She spotted the leather gloves on the floor.

But the Doctor was gone.

~*~

There were guns. Guns and explosions and it scared him. He hated guns.

When the man had entered with his weapon and the loud noises began, he ran. In the back of the room he found a window and slipped out, leaving that strange place and seeking refuge in the streets. He didn't know what he had been doing there but he sensed danger. And so he ran.

He quickly realised as he slowed that he felt no safer on the street. It felt wrong, like he was supposed to be somewhere else, but he couldn't remember where. People stared as he stumbled by muttering and confused. He hid his face with his coat and kept to the side streets and alleys where he was less likely to attract attention. The man with the gun might still be looking for him. Any one of these strangers might have a gun.

"Do you need help, mate?" asked a passerby.

He shook his head and kept walking. Someone asked him his name, but he didn't know it, so he didn't reply. In truth, he suspected that he didn't even have a name but he wasn't sure why he thought that, just that it felt right, in the same way that this place felt wrong.

His stomach growled. He was hungry and wondered when he had last eaten but didn't know. Could he ask one of these strange people for food? Was that how it worked, or was there something he needed first? He couldn't remember how one got food.

The streets were endless, winding around and around like a maze. He walked until his feet hurt and the once lit streets grew dark. He was getting tired, too tired to stand, he needed some place to lie down. But nowhere felt safe. The alleys were too dark and he still sensed eyes upon him. He needed to take refuge in the light. Then he turned a corner, and he saw one.

This street had a light, a small white lamp atop a blue box, casting a heavenly glow on the alley. He didn't know why but he instantly felt safer here, the feeling of panic subsided. The box was locked, so he collapsed in front of it and was quickly asleep.

He awoke with a start to someone touching him.

"Leave me alone!" he shouted to the stranger with the ginger hair crouched in front of him.

He sat up and scrambled back against the comfort of the strange blue box. He didn't know why but he was desperate to get inside it, it felt like the only place that would be safe.

"It's all right," she said in a soft voice, "I'm Donna, a friend."

"Do you know who I am?" he asked her, terrified.

"You're the Doctor," she said.

"Doctor who?" he asked.

The woman just smiled and offered her hand. "Just come with me. I'm going to help you."

He didn't know why he trusted her, but he did.

~*~

Donna had brought the Doctor back to the lab just as Professor Ben had finished making the antidote. He was a bit like a frightened animal himself at first, but after a round of injections, some food and rest the Doctor was entirely back to normal by morning.

Later that day, after passing every cognitive test with flying colours, Professor Ben pronounced him cured. Before leaving, the Doctor told Donna he had one quick errand to run in the TARDIS and to wait for him at Ben's lab. A short while later he returned with a parting gift.

"I wanted to thank you for your help, Professor," the Doctor told him, "so I thought you could use this."

The Doctor brought his hands around to reveal a small travel cage containing another chansatt.

Professor Ben was overcome, swiping at his eyes before taking the carrier from him.

"Yours is a female," the Doctor explained. "This one's a male. A quick trip to Dolia, I modified the sonic homing device to locate their habitat. You were right, there aren't many left. I rescued this one from poachers. Now you can breed more, and save them from extinction."

Ben nodded, "I will do it in memory of Hal Mon. He loved these animals. He would have been pleased. Thank you both."

They made their goodbyes and then Donna and the Doctor returned to the TARDIS together. Donna felt a bit awkward; the Doctor hadn't yet talked to her about what he'd been through and she wasn't sure if he would. It was more his way to simply move on without looking back. So she was surprised when at last, the Doctor spoke, still hunched over the console, fiddling with dials as they prepared to leave.

"The TARDIS must have sensed that I was lost and drawn me to her for protection," he said.

"Lucky it did or I might never have found you otherwise," said Donna. "I didn't know where else to look so I just took a chance that you might have come here, even without knowing why."

Another awkward silence followed.

"It's good to have you back," Donna told him finally, "I can't imagine what that must have been like for you."

At this the Doctor looked up. "Thank you for helping me. I don't ever want to forget you again Donna."

"Oh you won't, Spaceman," she said smiling, "I won't let you."