It's just a typical day at the Mystery Shack. Feds everywhere, Grunkle Stan's on his way to jail- wait, what?!
That's right, apparently Grunkle Stan stole some kind of nuclear waste from a Government facility- and in an effort to clear his name, Dipper and Mabel discover their Grunkle is not who he says he is?!
Stan had high hopes- he'd gotten free of the federal agents after a gravity anomaly gave him the upper hand, but luck was not on his side when a chance swipe of Deputee Durland's pinata stick hit him smack dab on the head. THUD! He was taken by surprise. Durland ripped off his blindfold with glee, certain he'd find the ground covered in candy. Instead there was just an escaped prisoner- wait, how did he get there? Just then, the feds rushed out of the interrogation room. As two of the officers rushed to detain Stan, who had been just a few minutes short of reaching the double doors, Agent Powers puts a hand on Durland's shoulder. "Good work, deputee. The prisoner would've gotten away had it not been for your heroic act."
"Heroic act?" Durland echoed, still a little surprised with what all just occurred.
In that moment, Agent Trigger's cellphone went off. "Agent Trigger speaking," he answered. "The chopper won't be coming? This prisoner is a slippery one, we should head to Washington immediately- I understand. Agent Trigger out." He hung up the phone and was silent for one dramatic moment. "We'll have to keep the prisoner in a local holding cell until conditions are safe for flying. It seems the anomalies are creating a magnetic field similar to the Bermuda triangle. No one goes in or out until we can determine the source." He throws Stan an accusing glance. "Make sure he doesn't get away again."
Stan glares back. "Hey, I haven't even got a fair trial," he protests. "You can't just throw me in jail! I'm innocent! Check the security tapes! You've gotta believe me!"
"You're really going to say that after resisting arrest," Agent Powers deadpans.
"I- I just wanted to check on the kids! After you two meatheads lost them, they could be anywhere! Can you blame an innocent old man who wants to make sure his kids are safe?! You're going to throw a poor old man in jail based on false accusations-"
Agent Powers put a finger up as his phone was now going off. A frantic voice was calling from the other end. "We've got proof Stan is innocent!" said the excitable young boy. "Yeah!" A girl interjected. "We've got video proof that Grunkle Stan was stocking the shop last night! So you buttheads better let him go!"
"Oh, really," Powers replied, unconvinced.
"You've got no case!" Dipper squeaked happily, as he and Mabel watched their grunkle stocking the Mystery Shack on the television screen. A clicking sound could be heard in the background. "But if you want, I've already come up with a list of potential suspects."
"If this is true, we will have to let your uncle go. ("Yes!") But we need that copy of the tape. Tell us your location and we will meet you there."
"We'll be at Greasy's Diner. Bring Stan." Dipper hung up.
"The call traces to the Mystery Shack- but the special ops guys said they swept the whole area," Powers observed suspiciously. "The boy's right," Agent Trigger said. "If there really is evidence that the old man was there last night, then we really have no right to detain him." Just then, the door burst open. "We came across a foul smell in the shack," an agent announced, "And we found body parts in the suspect's couch!"
"That's probably from the zomb-" Agent Trigger almost said, before Powers quickly shushed him. "This is serious," Powers announced. "It would be negligent of us to not take a potential murderer in."
"There was a zombie attack, you crooks! You saw it with your own eyes! I'm innocent! You can't-" Stan protested- but it was too late. Next thing he knew, he heard the familiar sound of cell doors slamming behind him. "Well, well, if it isn't Stanford Pines," a sweet voice came from behind him. "It's so nice to see you again- behind bars."
Stan wrinkled his nose. "Gideon. Great. I have to put up with this now?!"

"They have to let him free now!" Dipper said excitedly, beaming at Mabel. "This is solid proof! He hasn't moved an inch the whole time!"
"Yeah... The whole time..." Mabel said uneasily. "I mean... He's standing really still, isn't he?" Then, with a forced laugh, she added, "No wonder why his back's always stiff, amiright?" Dipper rewinded to review the tape again, pausing at a moment he hadn't noticed before- the tape cut out for just one split second before they saw Stan standing behind the counter.
"No, Mabel, you're right," Dipper agreed, smile fading. "Something's off. It's almost like he's frozen or something."
"Maybe the film is stuck?" Mabel asked. "Try fast-forwarding."
Several minutes. Several hours. Stan hadn't moved, but the room had gotten darker.
Mabel looked worried. "Maybe he's just standing abnormally still?"
"This is weird," Dipper said gravely. "Even for Stan."
Just then, the video caught a tremor in the earth. Stan in the video slid sideways, and then fell out of sight. Their suspicion was confirmed when a head rolled out from behind the counter.
"Could it be... Wax Stan?!" Mabel whispered. They exchanged worried glances. "I thought he melted," Dipper said. Mabel poked his cheek. "You of all people should know I rebuilt him. Grunkle Stan practically begged me to. He even bought me pancakes."
They stare at the screen in silence, then fast forward several hours. Dipper was getting agitated. "Why would he leave wax Stan out in his place unless he needed an alibi? What if he really was lying, and those agents were right? Maybe he really-" They both sucked in a breath when the tape cut again. When the picture returned, wax Stan was gone, and the real Stan was at the counter, reading a book.
Nothing seemed to add up. Dipper squinted at the picture. "Is that a book on physics?" he asked. "I thought he only read those Gold Chains for Old Men magazines," Mabel whispered in awe. "Maybe he's secretly studying to become a magician!"
"Mabel, that doesn't even make sense."
"Well, maybe we can show those agents part of the tape? We'll just make a copy that doesn't include his head rolling off?"
"But that would be lying, Mabel! What if Stan really is a crook?! We need to get to the bottom of this!"
"Dipper, wait!" Mabel ducks down to look at the box beneath the screen. "Maybe he left an explanation for us in here!"
"I doubt it. He wasn't exactly planning to get arrested. And besides- hey, what the heck is all that?" He lifts a fake ID with wide eyes. Mabel, equally concerned, looks back at him.

Suddenly a phone next to them goes off. "Hey, they told me I get one phone call," Grunkle Stan said on the other line. He sounded agitated- but that wasn't entirely surprising. "Uh, so I don't know how you guys managed to find a way to clear my name, but now I'm being arrested for the zombie parts in the couch- apparently murder's illegal now, or something. Anyway, tell Soos to get you some bus tickets. You two are going home."
"We are not going home," Dipper replies firmly. "You have a lot to explain, Grunkle Stan- if that is who you are-" Stan sighed heavily. "Kid, I know this is all very confusing, but I need you to trust me, all right? And call me if Soos... Sees anything unusual." An awkward pause followed. "Anyway, finger painting's starting in a minute. Apparently I have to participate. Uh- and one more thing. Quit being so paranoid. And don't snoop through my stuff."
"Grunkle Stan-" Dipper said, but it was too late, and only the dial tone responded.
"How can we trust him now?!" Dipper fumes as Mabel sifts through the evidence of their not-Grunkle's double-life. "And why the heck would we go home at this rate?! He's hiding something big!"
"Dipper, I found some kind of code thingy while you were on the phone," Mabel said quietly. "I... Think it's somehow related to the vending machine."

Stan hid his face in his hands, but he really wasn't given any time to brood. Finger painting was held in the mess hall, the inmates were gathered around the table, intent on perfecting their art. That is, until a honeyed voice piped up. "Standford, so nice of you to join us." Gideon, the ends of his nubby little fingers covered with blood-red paint, beckoned him over. Stan frowned, readying an insult. One of the inmates pulled out a chair and glared with one scarred eye at Stan, silently promising the consequences if he refused.
Stan swallowed his pride and accepted the chair. Gideon smiled in devilish delight. "Everyone, I've already told you all about the delightful Stan Pines, the man who sent me to prison." Inmates trained their eyes on Stanely, grumbling and spitting and pounding their fists into their hands. "Hoo boy," Stan said, fairly certain he'd already made many friends. Should've known the little troll would have a cult developed in here. Looks like there'd be one way towards a fast prison break, and he didn't like it. "Gideon, I think it's time we put our rivalry aside and looked at the bigger picture here. You're in prison, I'm in prison... Why don't we look into fixing that."
Gideon giggled monstrously. "I always knew you to be a fool, Stanford, but I forgot just how delightfully dense you are." He stood atop his seat booster and leaned over close to Stan's face, so he could whisper, "Why in heaven's name would I help you escape when I'd always dreamed of seeing you rot in prison? I don't need your help, sugar plum. If you want my help, you better learn your place around here, and fast."

Dipper and Mabel ran haphazardly towards the vending machine, where Soos was guarding it vigilantly. "Soos! We need to use the vending machine. It's important," Mabel said. She held up the code, and Soos surveyed it with slight confusion. "Grunkle Stan- I mean, whoever we've been living with- he has some kind of doomsday device down there," Dipper explained frantically. "We need to investigate before it's too late!" Soos shook his head. "I normally believe your conspiracies, dude, but I knew Stan for years. There's no way he'd be hiding something that big from me. He may be a crook, but he isn't secretly some kind of evil mastermind. Believe me."
"Soos, if that's true, then all we're doing is taking a peek," Dipper rationalized.
"Yeah, for all we know, it could be a code to infinite snacks," Mabel added.
"I don't know," Soos mumbles uncertainly, wringing his hands together. "Stan told me to guard this vending machine with my life."
"Stan isn't Stan, Soos," Dipper said with the last of his patience, but Mabel had already dove under Soos's arm and put in the code. Soos pulled her away from the machine just as the door popped open and bumped him on the butt. He turned around, miffed, still holding onto Mabel.

Stan stared at a rinsing cup at the center of the painting table. They'd taken away his watch, but he had a good feeling the time was soon. He grimaced as the water began rising from the cup, and the inmates floated away from their chairs. It was happening- he wasn't there.

Dipper was the first to reach the bottom of the stairs, Soos and Mabel trailing behind him in open-mouthed wonder. "It's the nuclear waste," Dipper said breathlessly, staring at the tubing in the walls. "And the journals- HE HAD THE JOURNALS?!" He assembled the books and passed the blacklight over them. "This device will tear our universe apart?! Guys, we have to shut this down!"
But they all had little time to react- gravity was failing them, and they felt themselves being lifted from the floor. Dipper quickly snatched up the closest journal with one hand and grabbed onto Mabel with the other. They could see through the glass that a glowing light was emanating from the next room. "Quick, find a cutoff switch! Anything!" Dipper shrieked. He flipped open the book and scanned the room. "Dude, I think I found something!" Soos said, swimming over to a manual override switch. "That's it!" Dipper said- but there was only a few seconds left, and Soos had to grab onto the nearest pipe in order to not be sucked into the portal. The twins clung to each other tightly and watched as the timer went down to three... two... one...
With a sudden burst of energy, they were all thrown back into the control room. Disoriented, they barely noticed a figure emerging from the portal. "G-Grunkle Stan?" Mabel asked uncertainly.
Dipper stands to shield her, still clutching one of the journals to his chest. "Don't come any closer!" He says in the bravest voice he could muster, squinting at the light. His heart was pounding in his chest, images of a triangular being flashing in his mind. "I don't know how you got out of prison, or grew out those sideburns, but until you give us some answers, I'm not letting you take another step!"
The man held up his hands. "Now hold it, boy, I'm not sure what's happening myself, or how you two kids found your way down here. I've just been pulled from another dimension, so if you're confusing me with someone else, I guarantee you I had no part in whatever crazy thing he did."
"Dude, yeah, you look just like Stanford Pines," Soos said. "A more apocalyptic version of him, anyway."

"Yeah, and ten times nerdier," Mabel agreed. "But seriously, what the heck is going on?!"

"I am Stanford Pines," The man replied irritably. "And I've never met any of you in my life! How on earth do you know my name?" He squinted and moved a step forward, focused on the journal in Dipper's hand. "My journal? How did you get ahold of this?" He reached out for it, but Dipper backed away, holding it more protectively.

"No! Not a step closer, Grunkle Stan—I mean, whoever you are!" Dipper shouted. After fighting Bill and a shapeshifter for those journals, he wasn't about to play the patsy. Especially with an ex-conman. "You're not getting these journals until you prove you're not lying! I'm not letting them fall into the wrong hands again!"

"I could say the same," Stanford replied, miffed and apparently a little offended. "I don't know who started this portal, but it was incredibly risky. As it is, I will need to contain the damage."

"All right, I think we can all agree it's been a stressful day," Mabel interjected, stepping in front of her brother to hold out her hand. Standford took it and shook it congenially. "Sorry for my brother. Hi, I'm Mabel. This is my brother Dipper. Over there is our good friend, Soos. Can you just tell me what the heck you were doing in that doomsday doohickey?! And why you look just like our Grunkle Stan?!"

"It's a long story," Ford said, incredibly reluctant to recount it at the moment. "And I have a few theories as to why I might look like your—wait, did you just say grunkle?"
"Great uncle," Mabel explained quickly.
"Oh, I see," Ford said, mentally cataloguing the modern phrase before he paused at the implications. "So tell me, how did you children find your way down here?"

"Also a long story," Dipper said, almost sarcastically.

"Right, I see. Now where is this – err—grunkle of yours?"

"He's in jail," Mabel said. "Mostly because he stole nuclear waste, among other things."

Meanwhile, Stanley was wiping a pallet of paint off his face while several other inmates recovered from the wreckage. "It's happened," he mumbled to himself. "I wonder if it worked."

"Aghh! Paint! Paint in my hair!" Gideon fussed, trying to wipe it off before the color dyed.