Mabel was circling the room slowly, keeping her eyes on her increasingly despair wracked brother, who circled her back with trembling hands gripping a crowbar. "Is Mabel still around somewhere Bill? Watching you parade around in her body for three years as a helpless ghost? Or have you wiped away her consciousness by now?"
"Dipper, please! I'm not possessed by Bill!" Mabel said defensively, her arms held out with palms up.
"ARE YOU SURE ABOUT THAT SHOOTING STAR?" Bill's voice rang out inside her head as the world slowly melted into chaos within her perceptions, the colors of every object beginning to run like paint, pooling in puddles across the ground and spawning horrors. As Bill warped her world from inside her mind, the only constant was Dipper staring her down, crowbar in hand and eyes alight with hatred, for even his twisted mind could generate nothing worse.
In a surge of motion preceded by a barely breathed apology, Dipper struck forward with his crowbar, attempting to bash Mabel over the head with one motion so he could drag her into the machine right after. The blow was slow, clumsy, and jittery however, and Mabel escaped by diving sideways to the floor, her sweater showered with spark when the crowbar smashed an unrelated computer.
Sprawled out on the floor, Mabel wanted to get on her feet and run for the elevator, but she found her rise from the floor slowed by a twist to the ankle she sustained on the fall, so instead she resorted to scrambling backwards on her hands and feet away from Dipper.
"How'd you do it Bill?" He demanded to know, slowly advancing on his sister, crowbar still in hand. "How did you stay hidden in there for so long, under everyone's notice!?"
"I'M NOT BILL, PLEASE DIPPER, I'M YOUR SISTER!" Mabel yelled back, fear and begging in her voice.
"LIAR!" Dipper screamed, messily slamming the crowbar down but missing the prone figure by a mile as Mabel scooted backwards. "Your cruelty betrays you Bill! I had never even considered the possibility but I should have known it from the bubble! My twin sister would never have been so cruel as to produce a horrible imitation of me and rub in my face how much they preferred them over the real me! Only your sick imagination could have thought to do that to me!" Deeply repressed hurt bled through every word, memories of being made to feel worthless by the person Dipper treasured more than anyone else in the world.
Mabel froze up in terror, flashing back to her "more supportive" version of Dipper that she'd paraded before the real thing back in the bubble while a guilty chill encased her heart, that Dipper had such a high opinion of her that he attributed her misdeeds to Bill Cipher. Her panicked backing came to an end as Mabel's head collided with a wall, and in a flash she threw both arms over her head and closed her eyes. After a long, dread filled moment however, the crowbar never fell, and Mabel slowly opened her eyes and looked upon Dipper.
Her twin was just standing there, openly crying with the crowbar held limply in hand, looking down at his sister being more terrified then he'd ever see her. After a stare down that felt like forever, Dipper simply fell to his knees, crowbar clattering to the ground.
"Damn you Bill." He breathed with defeat. "I won't give you the satisfaction of begging for you to make it quick."
Mabel looked on her brother, overwhelmed by a confusing mixture of fear, guilt, sorrow and regret. Even when his own life was on the line, even when he was afraid he'd be at the mercy of Bill Cipher, he can't bring himself to hurt his sister.
"And I can't even be honest with him." Mabel thought ruefully, then climbed to her feet. Dipper winced in anticipation as she approached him, but the girl kept on going, depending on her memory of the room to brave the illusion, and sat herself in the MI/ND Device, bringing the helmet down and closing the leather limb straps that had gone unused during her last sit. "It's... it's okay Dipper. Run the machine."
Wary that this might still be a trap, Dipper rose from the floor, and moved slowly towards the control panel, watching Mabel all the while like a big cat ready to pounce. Then, when he could touch the controls by blindly jabbing his arm out sideways, he contemplated the situation, then decided that if this was a trap he had no option but to spring it, and spun around like a flash of lightning, plugging away at the computer while tensing for a knife in the back.
The memory began to play on screen again, but this time Dipper was ready, and quickly routed the workings to another screen, allowing him to proceed with purging Bill Cipher from inside his sister. The Time Agent's deceptively warm voice began to filter through the computer's sound system, filling Dipper's ears with words he never imagined, and soon enough his work was complete, meaning all he could do was watch as the now gently crying girl waited for her mind to be free.
Back above ground, the battle against the undead had begun in force, with the previously still horde surging forward in motion, unleashing a synchronized wail of what seemed like rage as soon as Preston's body was shot down.
The Shack's automated security downed a few of them, electrified surfaces burning away the nervous systems Bill's magic puppeted to animate them while Grunkle Stan continued to fire at them from the upper roof, but their sheer numbers let them burst down the door after a bit. Grunkle Ford already had his pocket gun at the ready, and was firing into the crowd downing zombies while climbing backwards up the stairs.
He was soon joined at the top of the stairs by his twin brother, and together the two Stans stood against the zombie hoard, firing their weapons over and over into what seemed like a sea of dead flesh slowly advancing up the stairs, held back only by the kinetic impact and torn muscles of the raining bullets.
Eventually though, they did run out of ammo.
The two looked down at their guns clicking harmlessly, then up at each other. "We need to get to the attic!" Stan stated.
"What about Pacifica, where'd she get to?" Ford responded, only for the answer to come as the sound of rapid footsteps and a female yell as the girl in question ran up behind the twins and clumsily hurled a bowling ball she'd found down the stairs, the heavy object slamming through the crowd of flesh with the power of gravity fueling it, smashing through numerous cadavers in a sickly fountain of putrid tissue.
With the three together, they retreated to higher ground, having no choice but to hope the younger twins would be safe underground. Fortunately they managed to retrieve more of Stan's guns as they got farther up, keeping the hoard back, but eventually they'd be stuck on the roof, and they simply couldn't kill zombies fast enough, especially since, barring severe anatomical damage, many zombies could get up from their bullet wounds and continue pursuit.
Ford wracked his brain for a solution to this, but abruptly figured it out as he watched Pacifica and Stan attempt to escape out a window, their process briefly slowed when the old man stopped the young heiress, pointing out the armed electrified metal plate built under the window, designed to electrify invaders using this window as an entrance.
Urging the other two out after using a nearby emergency switch to disarm the trap independent of the full system, Ford abruptly began to shoot the floor of the shack, drawing confused cries from his two cohorts. The scientist began to grin however, when he struck metaphorical gold: a bullet grazed the plumbing, which began to leak water across the floor. Adding a few more bullets to speed the growth of the puddle, Ford put one foot out the window and rearmed the electrical plate on his way out.
The zombies soon kicked down the door, but were gradually eradicated as their attempts to shuffle forth and kill Stanford drew them into the electrified puddle, destroying their nervous systems and ending Bill's means to puppet them. Ford remained in place on the roof to maintained their attention, while Stan and Pacifica climbed down and flanked the zombie hoard, cleaning them up in short order.
Now, all that was left was to check the basement.
When the three stepped out of the elevator they found Dipper having taken a seat in a chair, body lax seemingly from shock, while Mable still sat in the chair as the machine simply hummed away. Neither were looking at each other, and the computer was playing the footage of Mabel's forest deal on repeat, Bill's final attempt to torment the Pines family as the weakened, passive section living inside but not controlling Mabel's mind was burned away.
With an eagerness to be doing anything else but be in this room, Dipper addressed the newcomers. "Hey guys, how are things going up there?"
"Bill attacked the Shack with an army of zombies." Ford said bluntly, then looked closer at the screen. "Dipper, what is going on here?"
In an uncharacteristic moment of silence towards his great uncle, Dipper simply looked at Mabel once and opted to run to the elevator. "Dipper, wait! There's a bunch of corpses up there!" Pacifica called out and went to follow him, leaving the Stan twins alone with Mabel.
Ford approached the machine to finish the work Dipper was doing, but came to a stop as he watched the scene repeat, a difficult to read expression on his face. When he'd seen the entire thing, he turned towards Mabel and simply asked. "Is this true?" he asked, having to speak somewhat loudly in order to be heard over the replaying memory.
As the recording reached the point where Bill emerged from Blendin's body and laughed over his victory, Grunkle Stan surged forward, ripped the computer's speaker out and furiously smashed it against the wall, allowing silence to reign again.
Mabel only nodded, refusing to look her great uncles in the eye over the emerging truth. To her surprise though, she felt a six-fingered hand sit gently on her shoulders. Stanford struggled to put his words in order, but eventually said "I won't mince words with you Mabel, you made a mistake, one I'm certain you are well aware of the consequences of. But, I made mistakes just like yours a lot more often, and nobody is more familiar with Bill's ability to manipulate others than me. It's not my place to judge you and what you've done, but you do understand that you have to talk to Dipper about this, right?"
Mabel looked up at her Grunkle Ford, eyes puffy and red, and nodded resolutely. "I understand Grunkle Ford." Then, she looked across the room at his twin. "Grunkle Stan?"
Stan looked at her with a moment of confusion, then snorted that "If you expect me to pass moral judgement on anyone pumpkin, you're looking at the wrong old man. Yeah you did something wrong, but I've done much worse things for much worse reasons all over my life!" Then his tone softened a little. "But Ford is right. You absolutely need to talk this out with Dipper. Just, not right now. Best let him cool off for a bit."
"That's well enough then since I need to finish scanning and clearing your mind." Ford added while beginning to work the control panel. "Bill explained a bit about how he was alive up above, but I think we can learn a lot more with a look inside your head."
The dawn eventually soon broke over the Mystery Shack as its inhabitants continued to work from night into day. Dipper and Pacifica left right away to alert Mayor Cutebiker to what had happened, quickly acquiring assistance in cleaning up the felled zombies and returning them to their graves, the whole town being always eager to erase evidence of Bill's existence.
Pacifica, who was sullen and quiet while working, was not terribly surprised when the police car that had been sent to her home discovered her mother dead. Bill's sadistic nature precluded any other possibilities. Not long after, she slipped away, and Dipper lost track of her in the rush to get all the corpses cleaned up as quickly as possible.
Down in the basement, Stanford worked through the data he had collected. Bill Cipher was indeed inside Mabel's head, but had been swiftly purged by the machine after an extensive scan of all data needed to work out what exactly happened.
"I'll tell you the good news Mabel," Ford began. The girl in question had gotten out of the chair awhile ago. Though she'd been awake for almost 18 hours at this point, she felt no desire to go to sleep, and had remained underground with Grunkle Ford when Grunkle Stan went up the elevator to help clean up the corpses. "Bill was inside your mind since you left Gravity Falls those years ago, but based on your brain chemistry I can safely say he was dormant until your return to town a few days ago."
"So I'm still me then, right? Those years I spent back in California, I was still myself?" She asked, and Ford nodded in affirmative. Mabel wasn't sure if that made her feel good or not.
"The Bill that was inside your head, as well as inside Preston Northwest and this time agent, were simply pieces of Bill, which he split apart in order to escape death and attempt to rebuild himself at a later time." Ford's hand began to stroke his chin. "In addition to his power, I suspect this also strongly reduced his intelligence. This invasion of the undead was sloppy and impatient in comparison to his previous scheming."
"Oh, three pieces, triangle, I gotcha." Mabel replied. "So, he'd dead for good now, right?"
"We can only hope, but I'm going to keep running tests to make sure." Ford stated. "In the meanwhile, I think you know what you need to do."
Mabel nodded and made her way to the elevator. The time she'd spent below ground had been put to good use by those working above, as the concerned citizens recruited by the mayor and directed by the town's exceedingly cheerful funeral providers had made swift work of the assorted corpses. While the contents of the Mystery Shack were still a mess and the whole room stank to high heavens, the obvious evidence of supernatural foul play had been cleaned up posthaste, allowing Mabel to search for her brother without being knee deep in the dead.
Despite her efforts however, she would not locate Dipper by her own efforts. Shortly before his twin had come up from the basement, the boy had concluded his own search, having found Pacifica Northwest sitting on the Shack roof, looking towards the morning sun.
Completely unsure of what to say in the face of this tragedy, he simply said "Hey" and sat down next to her. The air was exceedingly silent and awkward for an agonizing period of time, until Pacifica broke it with a deep sigh.
"I'm sorry Dipper."
The boy looked at her with confusion. "What do you mean Pacifica, you have nothing to feel sorry for." Then, when he realized how callous that sounded in this context, Dipper slapped himself in the face and began to stutter apologetically. "No, wait, I mean yes you have every right to feel sorry right, I meant that you have nothing to feel, well, responsible for, I mean..." His stammering ended with a groan of frustration. "This is all MY fault anyways!"
"No it's not!" Pacifica cut in firmly, snapping her eyes towards him with a hard expression. "Nothing that happened here is your fault, and I know it Dipper! You can't blame yourself for any of this!" Then, her tone softened with regret and she looked at her feet. "Anyways, I'm sorry I failed to live up to your example. You've spent so long showing me to be a good person, a selfless person, and none of it's stuck."
Dipper could only look at her in confusion, struggling with his own feelings at the moment. The blond simply let out a sigh and explained. "Both of my parents just died Dipper, and I'm not feeling what a good person should feel. I've known they were rotten since the night in the mansion, and they've only gotten worse since we lost it, but even then I know I should still feel bad about this but..." She threw up her hands in exasperation. "I don't know how I'm feeling! I'm feeling worse about the fact that I don't feel bad about my parents dying then I feel about my parents dying!"
Though still unsure how to best respond, Dipper made an attempt to assuage her feelings. "Well, there was a time when I hated Grunkle Stan for awhile and was going to leave him to die, but then I found out he was just doing his best to toughen me up, but then way later I found out her was actually repeating the mistakes of his own abusive parents that destroyed his relationship... with... Grunkle Ford..." Though he gradually trailed off and pulled his hat over his face when he realized how useless all that was.
Pacifica scowled at him for a moment, but then her expression softened. Dipper could be a tactless emotional klutz around her sometimes, but he always had a well meaning sincerity about him even when he messed up, and for a girl who'd been raised in a world of deception and social masks which she strove to escape from that meant a lot to her.
"Look, I appreciate the effort, but I think I just need some time to think this over alone, okay?" She asked of him, and Dipper wordlessly agreed, leaving the roof and returning indoors. Upon returning to the living room of the Shack, he ran into Mabel.
The two simply looked at each other for the longest time, a mire of emotions crossing both faces as they struggled to know what to say.
Mabel was the first to try and break the silence, stammering out "Dipper, I don't... I didn't... I'm sorry Dipper, I never meant to..."
"Do you trust me?" Dipper interrupted, and when Mabel looked up at his expression, she was struck by just how disappointed he looked. Anger she'd been prepared for, but this cut her to the core.
"Of course I trust you Dipper! You're the most capable, caring person I know!" Mabel cried out, tears beginning to form.
"Because I don't think you do!" Dipper accused, pain cracking his voice as he looked at the floor, tears of his own beginning to run. "You couldn't just let me be happy by myself so you ran away, tried to lock the town in a time loop, hid inside a world of fantasy while all your friends and family were dying, and you replaced me with a cheap copy. And all that..." Dipper breathed, choking on his own words. "And all that, I would have forgiven you for."
This was not the response Mabel was expecting, and she looked up at him with a small amount of confusion. Her greatest fear, made even strong after discovering the cemetery for Bill's victims, was that her brother would judge her beyond redemption.
"You were tricked by Bill at an emotional low point, and I'm in no position to judge you on that." He remarked, unconsciously rubbing a fork shaped scar on his arm through his clothes. "Heck, you didn't even know you were dealing with Bill at the time. I should have known better."
Mabel seemed to grow a smile at this prospect, albeit a faint one. "So, is that it then? Are we Mystery Twins forever?"
Dipper's next words crushed her. "No." He said simply. "Because while I can forgive you for all that, the thing I can't or just aren't ready to forgive you for is that you lied about it all."
Mabel's stomach dropped and she felt very small as Dipper looked right at her with teary eyes. "Why didn't you think you could tell me Mabel? Do you think so little of me that you assumed it would destroy our sibling bond? Or were you trying to manipulate me into staying with you back in California, away from my passion!?"
"I couldn't tell you because it hurt!" Mabel cried in response. "Everyday I was alone back at home I thought about telling you the truth, but the scorn I thought you'd feel for me, the rift it could create between us, I couldn't bare it! Everything else, all the hurt of growing up without my twin, the joyless grind of high school and the cruelty of everyone else there, I could take it all knowing you still loved me Dipper! Even just thinking that you hated me left me crying awake at night!"
Dipper was quiet again for a long while, trying to process how he felt about all this while Mabel quietly sobbed to herself. After a long think about it, he walked over and gave Mabel a hug of comfort, but pulled away before she could return it, disappointment still marking his face, but now diluted by pity.
"Mabel, you're my twin sister and the best friend I've ever had. Even though you're flawed, I know that you care about me more than anyone, even if you don't express it in the best ways. You're a bubbling, shining light inside my life, and every day that I lived in here in Gravity Falls I missed you too. You're my only sister Mabel, and I'll always love you."
"But, after everything I've learned today, I just can't trust you, and I don't know when I'll be able to again." Dipper said with regret, clearly wishing it weren't so. "Life is painful sometimes Mable, this life of truth and knowledge I lead even more so. Truth is paramount in this world, no matter how painful it is!"
"But this doesn't need to be your world Dipper!" Mabel yelled back. "We can home together, and just be normal teenagers and live normal lives away from all this terror and pain and death!"
"You know that's not actually possible Mabel. Not anymore." Dipper said solemnly, and then, when neither of them had anything else to say to each other, the boy simply pulled his hat over his eyes and walked away.
Time passed in a blur from there on out. It could have been days or just hours later, but soon enough Dipper and Ford were mounting up for everyone's worst fears: an expedition to another dimension.
"My analysis of the data confirmed Bill had correctly explained his method of survival." Grunkle Ford was explaining as the family was assembled before the portal, down in the basement. "However, he had told a critical lie, one I discovered by plugging the data extracted from Mabel's mind into the dimensional viewer device. According to that, there are nine pieces of Bill Cipher still out in the multiverse, lining up to the three dimensional form he achieved upon entering our reality."
"One one hand, this does mean that by deleting the fragment of Bill inside Mabel's head, I believe I've reduced his potential power forever. However, if three pieces of Bill reunite, he'd be restored to his original state, and he's still hugely dangerous in that form." Stanford continued while firing up the machine. "To that end, Dipper and I have returned the Viewer back to being a Portal, and will use it to track Bill Cipher across the multiverse and eliminate him, piece by piece."
"Ford, c'mon, this is crazy!" Stan protested. "I spent thirty long years trying to get you out of that thing, and you're looking to go back in and drag Dipper with you!?"
"The only other choice is to spend the rest of our lives with the threat of a returned Bill Cipher hanging over out heads." Ford replied, and that possibility even made the other grown twin nervous. "I have taken precautions to ensure we are not cast adrift however." He added, while pulling a small disc out of a coat pocket that possessed a simple metal switch on top of it. "This is a fast return switch, a device I invented that will remotely activate the portal, lock it onto the switch's location, and carry back anyone holding it. Me and Dipper will have a dozen each." Then, he put his hand on his twin's shoulder. "And before you say you're coming with us, I'm leaving you behind because I need someone I trust absolutely to keep the portal open, so we can return."
"Don't worry Grunkle Ford, me and Stan will keep this portal wide open for when you need to come back!" Mabel spoke up, then looked at Dipper. "You can trust me with this, I promise."
"I hope that's true Mabel." Dipper said a little cryptically, before the sound of the upstairs elevator opening caught everyone's attention. Pacifica Northwest had joined them in the basement, clad in the practical cargo pants/shirt/jacket combo she wore while on field work with the Pines family, with a bag slung over her shoulder and a nervous look on her face.
The room was quiet for a minute, then the newcomer spoke up. "So, uh, Soos told me you two are going to be gone for awhile. Dipper, Doctor Pines, can I come with you?" She asked, nervous but eager at the same time. "There's... there's nothing left for me here in Gravity Falls, and if I'm hanging around when my parent's disappearance gets reported I'll be shuffled off to a Northwest branch family on the other side of the world who only want what's left of our... my fortune. And I want to help you guys as much as possible before that happens."
"This is going to be difficult and dangerous. You understand that, right?" Grunkle Ford asked.
"What isn't when it comes to you Pines?" She replied jokingly, but then got a serious look. "It's about Bill, isn't it? He's still out there in some form and you're going after him I'm guessing? It's the only threat big enough that I could imagine that would convince you to do this, and I want him dead just as much as all of you."
"You have helped Dipper with his research before, so you're not completely unprepared." Ford mused, then differed to his apprentice. "Dipper, it's your call."
The decision was made, and then the two twins watched as the three figures stepped towards the active machine and dissolved into the light.