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Chapter Text

“Mr. Pines, how do you respond to allegations against your newest product: The Mystery Shack Mystery Milkshake?” The reporter asks, her cheeks hollow and eyes dull. Her eyelids droop while she tries to clear her voice, which is as hoarse as a chain-smoker, despite having been only the picture of health yesterday. She's like a hearth, beckoning you close even as the smoke rises. Literally. Black smoke emits from her mouth with every word, like her questions themselves are a poisonous haze. She puts a mask over the lower half of her face in an attempt at courtesy, but wisps of it leak out from the seams.

Dipper isn’t sure what’s become of her, there isn’t anything like her in the journals--he’s checked. Maybe that’s why this whole situation has him feeling so uneasy.

Normally, the blame for a suspicious Mystery Shack product would fall on its owner and manager, Soos, but Stan still owns the trademark, and had publicly boasted about The Mystery Shack's Mystery Milkshake (MSMM, for short) recipe being self-discovered and proprietary. When people who drank the MSMM began turning into monsters, it was easy to find the person responsible.

Stan says, “If the Mystery Shack Mystery Milkshake was really turning people into monsters, the boy and I would both have transformed already.” He pulls Dipper in closer, so the cameras can see him holding a half-consumed MSMM, “Sorry, kid—”

Dipper feels Stan take the shake from him and, despite having intended to not finish the shake, he reflexively tries to take it back, "Hey!"

Stan, in full view of the cameras, takes a long sip of the MSMM. He swallows, then opens his mouth to show he truly did drink it. “There, see? The only mystery with this milkshake is the flavor! If you haven't tried it yourself, then come visit the Mystery Shack and try it before it’s gone! The first person who correctly guesses the secret ingredient gets one free item from the store!”

He shuts the door, giving the shake back to Dipper. “Sorry for stealing your shake, but these damn reporters are like vultures today. Joke’s on them! After today, I’ll have doubled my sales!”

Dipper awkwardly holds the shake, “I don’t know. What if they’re right about the shakes? I mean, Soos, Wendy, and Ford have all transformed already—”

“Yeah, and you, your sister, and I have had just as much to drink, and we're all fine, aren’t we?” Stan cracks open a beer can, then looks at Dipper, “Want a sip? Fair’s fair.”

Dipper looks at the can skeptically, “Uh, no thanks.” He looks down at the shake in his hand, then sighs. He resumes drinking it, sucking down the sweet drink through the clear straw (Stan said the colored straws were too expensive). “Okay, but, really, the mystery ingredient is vanilla, right? It tastes like vanilla.”

“Nope.” Stan says, beer in hand as he takes a long sip. He sighs, satisfied, “The Mystery Shack Mystery Milkshake's Mystery Ingredient is just that, a mystery.”

“My head hurts,” Dipper says.

Stan says, “What, did you get brain freeze?”

Dipper doesn't get the chance to respond before he hears Ford hiss from the kitchen window, “Sssssstanley.”

Stan grunts in acknowledgement.

“Ssssinccccceee when have you had sssiiiixxxxx fingerssss?” Ford questions, sounding rather concerned. The hydra’s three heads crowd in the small window, trying to get a closer look at their twin. Amber eyes focus on the beer can, where Stan’s fingers are wrapped around it.

Stan’s six fingers.

Dipper feels his heart stop.

Stan is quiet for a moment. The silence doesn’t last long. “So what? People are turning into monsters, not my brother.” He rolls his eyes, “Really, it’s probably just a side effect from messing around building that portal for so long—”

The man’s rebuttal is cut off by a heavy thud coming from the attic.

“Mabel?” Dipper calls out, immediately moving to check on his sister, who had been upstairs getting ready for a party. When he makes it all the way to the ladder and she still hasn’t answered, he becomes panicked, “Mabel?!”

Stan is close behind him, while Ford watches helplessly from the yard, his three heads checking all of the windows they can reach. Dipper races up the ladder, pushing the attic’s door open to reveal…

A seal?

A round harbor seal lies belly up on the carpet, MSMM spilled on the floor beside it as it flails its flippers to roll over. It vocalizes loudly, making startled honking noises as Dipper scrambles to help it right itself. Stan follows Dipper up the ladder only to stop midstep at the sight before him. “Dipper, what the hell are you doing?”

“I’m pretty sure it’s Mabel!” Dipper answers, trying to roll his sister over as she makes even louder noises as if saying don’t you recognize me Gruncle Stan?! Her whiskers wiggle as she hyperventilates, cheeks puffing and deflating like she needs a paper bag. He huffs at the effort, “Help me roll her over!”

Stan’s knees groan as he joins his grephew in rolling his griece over. “I’m getting too old for this, kid.” He pushes on Mabel’s side, trying to roll her over. Finally, with Mabel thumping her tail and reaching with her flippers and both boys pushing, she’s able to land on the carpet belly-side down. They all slump in relief.

“Okay, so maybe it’s the Mystery Shack Mystery Milkshake,” Stan says.

Both Dipper and Mabel groan at that.

“Wait,” Stan says, “Why haven’t you transformed, Dipper?”

And he doesn’t know. He was the first to try it, and was one his third in as many days. Mabel lets out a distressed sound, and Dipper decides now isn’t the time to worry. For whatever reason, the people of Gravity Falls are becoming monsters, and it seems Dipper is either on borrowed time or immune. He can’t think about that now, not when he might be one of the few people who can actually stop this.

Chapter Text

Dipper paces around the room, trying to ignore the way his voice rises with panic—he has enough to overthink right now. “So, everyone in Gravity Falls except for me, including my sister, has become a monster because of a milkshake? Do I have that right?” He pauses, allowing time for a response.

“Yes.” Stan says.

“Gruncle Ford is a Hydra,” Dipper says, pointing out the window to where the man in question has stood for around six hours now, officially far too reptilian and far too large to enter the shack. “Mabel’s a…” Dipper has no idea, honestly. Gruncle Ford’s journals only described things he had personally encountered, and he’d never encountered a creature like Mabel Pines. “Mabel’s a seal or something.” Mabel, the seal, barks loudly, as though in agreement. Dipper continues, “You’ve grown extra fingers,” he says, pointing to Stan’s hands, which are wrapped around a second beer can. A squeal from the floor reminds Dipper, “Waddles is a scarecrow,” the straw-stuffed pig licks the milkshake from the carpet unashamedly, “and you won’t tell us what’s in the Mystery Shack Mystery Milkshake?!”

“Well when you put it like that…” Stan says, “yes.”

Dipper facepalms, ignoring the way his forehead stings from the impact.

“Damn it, Sssssstaney,” Ford hisses, “Jusssst tell ussss.”

Dipper nods emphatically, “We have to turn everyone back!”

“Well…” Stan says, drawing out the “l” sound.

“Gruncle Stan.” Dipper says seriously, “My sister will kill you if she has to go to prom in flippers.”

Mabel barks in agreement.

“Oh, fine, but you’re going to be real disappointed,” Stan relents, “The Mystery Shack Mystery Milkshake’s Mystery Ingredient is water.”

There’s a moment of stunned silence.

“Water.” Dipper says, disbelieving.

“Water.” Stan confirms.

Again, the Shack falls silent.

“Sssssstanley,” Ford says carefully around the fangs in his many mouths. “Would thissssss water happen to be locally-sssssourccccced?”

“Why, are you a cop?” Stan asks.

Dipper asks, “Stan, where did you get this water from?”

“You know,” Stan says nonchalantly, “Around.”

“Wasssss it near the old quarry?” Ford says, “Jussssst passsst the ‘keep out’ sssssignssss and the ‘Danger, may irreverssssibly alter your sssstate of being’ ssssignsss?”

“Yeah, actually, how’d you know?” Stan says.

Dipper says, “Stan,” voice dripping with exasperation.

“What?” Stan says, “When you get up there in years like me, that sounds like a good time.”

“I ssssshould have dessstroyed that lake when I had the chanccce.”

“What’s going to happen to us Gruncle Ford?” Dipper asks.

Gruncle Ford hisses in frustration, “That’sssss jussst it. I don’t know!” He explains, “I noticccced sssome anomaliessss in the area and I blocked off access so I could safely study it later. Damn you, Sssstanley!”

“How was I supposed to know?” Stan says, “Sure, there was a squirrel with another squirrel fused to its tail, but conjoined twins are a perfectly natural thing.”

“Gruncle Stan,” Dipper says, “Right now everyone in Gravity Falls has become a monster. My uncle is a hydra, my sister is a seal, and somehow you’re our best chance of figuring this out. Please, take this seriously.”

Stan says, “I don’t know, do we even really know that the Mystery Shack Mystery Milkshake is to blame?”

“Damn it, Stan! Forget about the Mystery Shack Mystery Milkshake and just show us what you did already!”

“Fiiiine,” Stan whines, “But you’re totally signing an NDA after all of this.”

Dipper reminds him, “NDAs don’t cover illegal activity.”

“Who told you that?” Stan says, looking very suspicious.

Dipper gestures to the seal currently flapping her fins like wings, “Mabel.”

Stan shakes his head in disappointment, “And here I thought Mabel would follow in my footsteps someday. Only a year with your folks and she’s become a snitch instead.”

Mabel yells loudly in objection, but Stan only sighs, “Alright, let’s go.

“This is it?” Dipper says, looking around, “This is what caused all of this?” He’s looking at what appears to be a perfectly ordinary lake. Frogs croak along the bank out of sight and dragonflies skip across the water’s surface. Reeds grow along the lake’s edge in dense clusters, and Dipper is 99% sure his legs are covered in mosquito bites, but—

“Yup. Pretty normal, right?” Stan pulls aside what looks like a hunter’s gilly suit, revealing a hidden cooler beneath it. He pulls off the lid, showing his nephew that it’s full of somewhat cloudy ice. “I collect the water, freeze it, and blend it with an ice cream base and BAM—Mystery Shack Mystery Milkshake.”

“Your brilliant idea for a mystery flavor was to water down ice cream?”

Stan says defensively, “Well you didn’t guess that, did you? Besides, it tasted like vanilla!”

Dipper groans, he supposes he should have known Gruncle Stan wouldn’t buy when he could creatively advertise. “Okay, so we just need to take this back to Ford’s lab and then we’ll—” The sound of rustling foliage nearby gives him pause. “What was that?”

A moment later and Mabel appears! The strangest thing is how normal she seems—how human. Mabel says, “Hey, guys, so funny story—”

“Mabel! How are you human again?!” Dipper asks—no, demands.

Mabel says, “Sooo I was getting ready for Candy’s party tonight and I was looking for something cozy to wear. Obviously, I pulled out a sweater from my closet.”

“Obviously,” both Dipper and Stan say, despite the fact that she’s currently wearing a T-shirt.

“Well, I saw a sweater I didn’t ever remember wearing so I tried it on to see if it even fit and suddenly, POOF!” She does an exaggerated explosion gesture with both of her hands, “I was a seal! I managed to teach Waddles how to help me take it off and then I was human again.”

Dipper says, “Wait, if this is all because of the water, does that mean the water can change things other than people? Did you spill the milkshake on anything?”

“Ugh, yes!” Mabel says, looking quite upset, “I spilled the one I got this morning on my favorite sweater and it was like totally ruined! I threw it in the closet for future me to deal with and—Oh.”

“Crap,” Dipper says, “so even people who appear immune can still turn into monsters with the right exposure.” He sighs, looking from Mabel to Stan and back again, “That means I can’t do any of the testing myself without risking infection, and Ford’s not exactly up to the task himself right now.” Stan opens his mouth, but Dipper cuts him off, “That means you two will have to do it, and I’ll have to teach you.”

Mabel crinkles her nose in disgust, “But I hate science.”

Dipper sighs, “Let’s just get this cooler back to the Shack, alright? Help me carry this.”

All three Pineses crouch down and haul the cooler out of its hiding place.

So, Dipper has to admit, he might not be the best teacher. Mabel’s smart, but she has no interest in following a procedure, and even less patient. Stan, on the other hand, is great at figuring things out as he goes, but he wasn’t exactly studying pathology or microbiology when learning how to bring his brother back. It’s been a long time since Stan was out of his comfort zone, and it’s clear that he’s had quite enough of it. Dipper knows that they could do it, but he’s not sure that he can teach them how.

Dipper’s in the middle of trying, again, to explain what they’re supposed to do when he finally says, “Alright, we’re done for the day. I learn best on my own anyway, so let’s all sleep on it and figure this out tomorrow.”

“But, we still don’t know—”

“Actually,” Mabel says, “I still sorta have that party to go to. Could this wait until tomorrow?”

“You’re kidding.”

Silence.

“The entire town, except for me, has turned into monsters and you’re worried about a party?” Dipper thinks he might be insane, but quickly decides it’s everyone around him who’s lost their minds.

“Most people think it’s kinda cool, actually,” Soos says, looking up from his phone, “Everyone is talking about it online. People are coming from out of town to try out this milkshake themselves!” The man has become more canine, with large teeth, fluffy sideburns, and floppy ears—the perfect mixture of the lovable Mystery Shack’s manager and the classic Hollywood wolf-man.

Dipper says, “We can’t—You can’t—” He groans in defeat, “You all are really going to pretend this isn’t a problem?”

“Yup.” Soos says.

“Kinda,” Mabel agrees.

“I never really got what the big deal was to begin with,” Stan says, “A couple extra fingers never hurt anybody.”

The others file out, leaving Dipper behind to deal with the crushing realization that, once again, he’s the odd one out.

“Tomorrow,” Dipper says to himself, but even he doesn’t believe it.

Chapter Text

“Tomorrow,” he says, when Mabel excitedly shows off her magic sweater that turns her (and just her, he’s tried wearing it himself only to find his skin suddenly very itchy) into a seal.

“Tomorrow,” he says, seeing Waddles start growing straw from his seams as he starts to look more and more like a scarecrow. The poor pig squealed in fright after catching sight of his reflection on more than one occasion, but he’d horrifyingly developed a habit of eating his own shed straw in an absurd act of self-cannibalism.

Stan discovers he’s something a little more than a twelve-fingered-Gruncle when he turns into Soos one day. He figures it out as he goes along, like he’s good at, and he now takes requests at the dinner table, shapeshifting into whoever and whatever amuses him. Dipper looks at a mirror image of himself and his twin before checking, “Tomorrow?”

They all promise, but he’s left alone in the lab once more when tomorrow comes. It’s been weeks and there’s nothing to do but seethe as he looks at the samples he can’t be exposed to, even indirectly.

“Marcus says I’m a selkie, Dipper, can you believe it?” Mabel says, “I googled it and, guess what? If someone takes my sweater, we’re like, totally magically married!”

Dipper startles, “What?”

“Yeah, it’s like some Scottish thing. Anyway, the point is, now I know how to tell if someone likes me! If I just ‘forget’ it somewhere, then I’ll know once they pick it up! It’s fool-proof!”

“What? No. Mabel!” Dipper’s voice does not crack. He urges, “We have to find a cure ASAP.”

Mabel says, “Okay, but can’t it wait until after I’m married?”

“No!” Dipper ignores the disappointed look on his sister’s face. “Mabel, you are way too young to get married and—”

“Lalalalalala!” Mabel yells, plugging her ears with her fingers to drown her brother out.

Dipper tries to pull her hands from her ears, but Mabel’s taller than him now, and she ducks away from his efforts easily. She laughs, rushing out the door before Dipper can get another word in.

“Tomorrow, my ass,” Dipper says pettily.

“Oh!” An elf asks him at Candy’s Summereen Party, “Are you dressed as a human? That’s pretty clever actually.” She sounds amused, smiling wide, “Summerween’s a little different when most of the costumes look like your friends and neighbors.”

“Haha,” Dipper says awkwardly, “Yeah.”

“Paisley,” the elf’s friend, a satyr, says, “He is human. It’s not a costume.”

“Oh!” Paisley says, looking apologetic, “I’m so sorry. I had no idea. I didn’t think there were many of those these days. You must be lactose-intolerant or something, right? Tough luck!”

Dipper says, “I’m immune, actually. I think so, at least.” He gave into the temptation weeks ago, drinking more of the water and even splashing some on his hat. When that didn’t work, he dipped the whole thing into the cooler, which had long since thawed. No such luck. “I’m just a human.”

“Oh.” Paisley looks around awkwardly, unable to look Dipper in the eyes, “Um, enjoy the party!” She excuses herself from the interaction abruptly, grasping her friend by the elbow and pulling her away and to the dancefloor. Her pointed ears are just long enough to be seen peeking out between her black curls as she walks away.

Dipper takes a sip of his punch, which has a (hopefully) fake eyeball floating in the pink-red liquid. Last summer, Dipper would never have even questioned the likelihood that a drink contains human body parts, but when the host’s mom is a vampire, one can’t be too certain. It’s while he’s thinking about this that he suddenly hears a voice in his ear, “You’re immune?” The voice sounds like sandpaper against styrofoam, and Dipper isn’t afraid to admit he nearly spit out his drink.

Forcing a rough swallow, he turns to face the person addressing him, finding the speaker to be a very short (even shorter than Dipper) elderly woman that stares at him intensely. “Yup,” Dipper says, looking to the side to avoid staring at the hair sprouting from the wart on her cheek. He wonders if he’d be so bothered by the sight of monsters if he was one himself.

The woman’s hair is long and gray, though it is clearly close to falling apart, judging by the amount of split ends. The woman’s nails are long and yellowed as well, though it is clear they have at least been clipped and filed somewhat recently. She wears rings on many of her fingers, though none appear to be a wedding band. She says, “You’ve got to be working on a cure, right?” She seems surprisingly vibrant for such an old woman, grasping one of Dipper’s hands with both of hers, “Do you have one already? Is that why you’re immune?”

Dipper jerks his hands away from her, “Uh, I’m in the process of—”

“Cut the crap, Dipper,” she says, “How can I help?”

Something about the blatant disregard for both manners and Dipper’s feelings seems to ring a bell somewhere in the back of Dipper’s mind. “Do I know you?”

The woman groans, “Yes. I get that I look different now but—”

Dipper looks closer at the woman’s face, trying to solve her like one of his mysteries. Her round face is on display with a handkerchief pulling her hair back and out of the way. Pearl earrings sit perfectly in the center of her lobes, betraying a need for perfection that would rival even Dipper. It’s when he notices that odd coloring of her eyes, a brown with a warm gold undertone, and a faint purple dusting on the lids that he figures it out.

“Pacifica?”

“Say it a little louder,” she suggests, “I don’t think the people in New York heard you.”

Dipper scrambles, “What happ—When did you—What are you?”

Pacifica says, “My best guess is a hag. If it’s all the same to you, I’d rather be a teenage girl.”

“Right, yeah, I mean… I should shut up.”

“Nuh-uh,” she says, “You should keep talking about this supposed immunity of yours. What do you know?”

“Not much,” Dipper admits, “Just that I’ve probably had more of this milkshake than anybody in town but none of the effects. If you’d follow me to the Shack, I could show you what I’ve found?”

“Please.” She says, clearly desperate for a cure, or even an escape, “If I have to spend another minute in Candy’s house waiting for the moment her mom decides to eat me, I might just die.”

Dipper snorts, “That is what happens when one is attacked by a vampire.”

“Less talking, more walking.” She pushes him toward the door, totally uncaring of the splash of punch that stains his shirt. His indignant hey goes ignored.

“So… I’ve become a decrepit, old hag—”

“All three of those words mean the same thing,” Dipper supplies, and is immediately ignored.

“—because Stan was too cheap to fund a gimmick? Oh, when I get the FDA on the phone, he’ll be finished!”

Dipper says, “I’ve tried calling them. They’re staying out of it since he technically didn’t get anyone sick and he advertised the ingredients as locally sourced.”

“That makes no sense!”

Dipper shrugs, “I’ve also sent some of the water samples for analysis at some nearby testing facilities. I also asked several universities for help, but I’m only thirteen so they won’t talk to me without a parent or legal guardian and…” he gestures at their surroundings, “Stan I think technically counts, but he’s not interested in anything but his profits and my parents haven’t picked up the phone since we left.”

“Oh my god are they okay?” Pacifica asks.

Dipper forgot, for a moment, that as awful as Pacifica’s parents are, they still remember she exists. “Yeah, they’re fine. They’re these kind of…” He struggles to think of a word for it, “They try to avoid us during the school breaks. They’re not really into parenting. They do all the things they need to when they need to, but once someone else has agreed to be in charge of us, Mabel and I are on our own.”

“That… makes a lot of sense,” Pacifica says.

“What do you mean?” Dipper asks.

Pacifica shrugs, “I guess I just understand you and Mabel better now. My parents never let me make a choice. Yours let you make too many.”

“I go back and forth on whether or not I think it’s a bad thing,” Dipper says, “I think I would have gone crazy if my parents were normal.”

Pacifica lets the subject drop, “So what did the labs say?”

Dipper pulls up the report, “The left column is the compound detected, the middle is the expected percent composition based on geographic location, and the rightmost column shows the measured percent composition.”

Pacifica eyes the screen in silence for a minute or two before coming to the same realization as Dipper, “The total percentages exceed the realm of possibility.”

“Exactly!” Dipper says, “We somehow have more matter and mass in the sample than the total initial sample mass.”

“The reports have to be wrong,” Pacifica says, “Have you heard back from the other labs? They might have had bad equipment.”

Dipper pulls up the remaining reports, “There’s slight variance, as can be expected when using different samples from a nonhomogenous mixture, but even so—”

“There’s still too much substance.”

“It completely defies the logic of the test and, unless there’s something I’m missing, several laws of physics and chemistry.”

Pacifica asks, “Have you tested tissues exposed to the water?”

Dipper says, “Only in myself. I’d need a control and a willing subject to—”

Pacifica interrupts, “I’m willing. What do you want? Blood? Skin? Hair? Spit?”

“Are you alright, Pacifica?” Dipper asks, hesitantly, “You’ve never really been interested in my nerd stuff before.”

Pacifica says, “I never said I wasn’t interested,” she looks away, crossing her arms, “but when your parents kick you out and you have to move in with the most irritatingly nice person you’ve ever met just because you look like you’ve clawed your way out of a nursing home, you become a little obsessive.”

“Wait.” Dipper says, “That’s why you were at Candy’s party? You live there?”

“Temporarily,” she says.

There are many things Dipper wants to say to that. Instead he says, “We have room. Here. At the shack. Ford can’t really use his room right now, I’m sure he’d understand if you stay with us.”

Pacifica looks momentarily surprised because she controls her expression again, “I’ll think about it. I’m not sure what’s worse—sharing a house with the guy who did this to me or having to convince your host family that your blood would taste bad.”