Bodyswap
The sled slammed into the ground, thrashing its passengers around, but not throwing them overboard. After they settled, Dipper steadily rose.
"What was that?" he said aloud. "Something came loose."
"I don't know," Mabel snapped back, climbing out of the sled.
Dipper moved past her, ignoring her mood and checking the systems. He discovered what had made the noise. A panel had opened, spilling some of the loose wires into the floorboard. The wires were too heavy by themselves; they would keep coming unplugged if the wires were left to bounce around like that. They couldn't make another jump with this as it is. He needed to get that panel back into place, but he needed a screwdriver.
"Okay," Dipper decided, "Easy fix." He looked over to Mabel, who was watching him begrudgingly with her arms crossed. "Just need to grab a screwdriver."
He looked around the area where they landed. "Hey," he noticed, "This is where the gnomes kidnapped you, remember? The Shack's not too far from here."
"Yippee," Mabel waved her finger in the air for false excitement.
"Come on. We shouldn't be more than a few minutes."
He started to walk away, but then Mabel said "No."
Dipper turned around, "What's that?"
"I said no," she repeated, "I'm done being pushed around, you go get that stupid screwdriver yourself."
"Mabel, what's going on?"
"We seriously couldn't take another five minutes in that last dimension? I just wanted to see a few things! When was the last time we did something I wanted?"
"Seriously?" Dipper started fuming, "How about the time I helped you and Mermando? Or when I gave up a summer with Wendy so you could have Waddles? Or, better yet, how about that time you promised to help me figure out the password for that stupid laptop, only to turn around and make a sock-puppet show for you and puppet loving Gabe!?"
Mabel turned her back, sat down and pulled the hem of her sweater down past her knees. Then she pulled the neck of her sweater over her head: Sweater Town was in full lockdown.
"Fine," Dipper said bitterly, "Hide out here, in Sweater Town, meanwhile, I'm going to be busy with SAVING OUR FAMILY!"
He stormed off towards this world's Mystery Shack, angrily muttering under his breath about his sister. The small cabin came into view, and he took a breath to calm down.
Alright; just focus, get the screwdriver without anybody noticing you were even there. They'd probably be looking for it forever. Dipper wondered if this is how he lost some of his toys over the years, someone else from another dimension stopping by and-
"Stop it," He told himself, "Focus."
He watched as someone exited, he was off to the side of the shack, so he couldn't see them directly, but he heard who it was.
"I have to go get some groceries, Sweetie," Grunkle Stan promised with sincerity, "Do you want me to get you anything?"
If there was answer, Dipper couldn't hear it. He waited patiently to see what would happen next.
"...Okay," Stan then said, "I'll bring you girls back some food. You be sure to call me if anything happens, okay?"
After another moment with no answer, Stan closed the door, locking it loudly. He soon got into his car and left, leaving the Shack alone.
Dipper walked up to the door. He remembered that, for a short while, the locks on the front door for the Mystery Shack were busted, and it took a couple of days for the guy Stan hired to replace it. Until then, he always made a show of locking the front door, like it would dissuade any lurking thieves. Dipper turned the knob and it the wooden door swung open with an ominous creak.
"Doth protest-eth too much, Stanford," Dipper parodied. Then he corrected himself, "Er, Stanley." He really needed to stop talking to himself.
He stepped inside and tiptoed through the dark gift shop. It had some different looking attractions from the Mystery Shack he was familiar with. They looked even faker than the ones he knew, like someone only put half the effort they should have into making them. Dipper snuck through it all, all the way to the new break room Soos and Wendy used. He spotted the tool box sitting there, right by the door as soon as he opened it.
"Okay, screwdriver... screwdriver..." He whispered to himself as he searched. No matter how quietly he tried to move, the tool box, and its contents rattled to cause as much attention as possible. Finally, Dipper found what he needed. "Gotcha." He strode confidently, but silently back to the front door, only to be stopped short.
"Dipper?"
He spun around, his heart skipping a beat. He'd been caught, red-handed. Mabel had appeared in the doorway. Her sweater was missing, leaving her in shorts and a tank top. In the loop of her pants was a familiar looking hat, and in her hand was a familiar looking gun with a light bulb instead of a muzzle.
The sight of Mabel like this rose too many questions for Dipper, and he spent a second too long staring instead of running. Mabel raised the Memory Eraser up at him with fresh rage and torment crossing her face.
"Bipper," she sounded angrier than any other time in Dipper's life. Anger didn't seem to cut it. He had never known her to strike out at other people, but this was a different world, with a different Mabel.
"Wait, wait!" Dipper held his hands up, and dropped the screwdriver in the process. "Mabel, its me!"
"You're not Dipper!" She protested. "Dipper's been missing for weeks now, so who are you!?"
Dipper had always been a terrible liar, so maybe he could reason with her. "Why'd you call me Bipper?" He asked, "What does Bipper supposed to-"
"It's Bill!" She yelled stepping closer. "You're Bill! He took over your body, and then I thought we got it back, but..." Her eyes brimmed with tears. "Give. Him. Back!"
"Mabel!" Dipper begged, "Look at my eyes! You can see them, can't you?"
"I don't-" she paused and took another two steps closer. Dipper didn't dare blink as she squinted at his eyes.
"You're...not Bill?"
"Mabel?" Dipper heard a new voice, "What's all the-?"
Dipper watched as Pacifica entered the room, and saw the two of them standing there. She rushed over to them, pulling Mabel back by her shoulders. Dipper saw the fear in her eyes. The way these two looked at him; it broke his heart.
"Mabel," Dipper tried to explain, "I'm not the Dipper you're familiar with. I'm…from another world, and I'm trying to-"
"You're not here to stay?" She interrupted.
Dipper blinked. If she heard him, she didn't seem to care.
"Are you here," she spelled out slowly, "to stay."
Dipper gulped, "No…I'm not."
Mabel shook her head, "Then it doesn't matter. Dipper's still missing."
She turned and walked away, dragging her feet out of the gift shop, and up the stairs.
Dipper was at a loss for words. He started with the most obvious question.
"What happened?" he asked Pacifica.
She looked at him skeptically. She looked down, at his arms.
"Dipper had cuts and bruises all over his arms," she noted, "You're really not…"
She made eye contact with him.
"Dipper made some kind of deal with Bill," Pacifica explained, "in Mabel's words; they beat him, and got your body back, but…" she shook her head, "You just…weren't there one day. You disappeared, and if you ever showed up, it was to cause trouble."
She coughed, then continued. "My family and I…had this house party, and at one point…you were fighting Bill, whoever that is. You…helped save us. I moved here with Mabel, to help her the way she helped me, and everyone else.
"She..." she hesitated, "She doesn't sleep anymore. She just...sits out on the front porch, staring out into the woods at night. She goes back to bed before daylight shows, so her Uncle doesn't even know about it... I just... sit with her..."
Dipper swallowed the sadness down. He needed to get out of here, before he did something stupid. He picked up the screwdriver and started to back up out the door.
"I can't stay here," he said, his heart catching in his throat, "My family back home is in danger, and I just…." He sighed, "Thank you, for looking after Mabel. It means so much that-"
She assaulted him with a hug.
"…I know that this isn't… totally right," she said, "but thank you, for saving us when you did."
She pulled back, and Dipper started to leave. "Pacifica," he warned, "Keep the journals away from Bill. He wants them destroyed, okay?"
She nodded, understanding.
Dipper then added, "Take care."
He left the Mystery Shack and sprinted all the way back to Mabel- his Mabel- sitting by the bobsled. He burst through to the clearing, and saw her standing up- the lockdown of Sweater Town had lifted- looking to see what was wrong. Her eyes were red from crying.
Dipper stopped in front of her to catch his breath.
"Dipper?" she said, confused, "What's wrong? You're-"
Dipper didn't notice his cheeks were wet with his own tears. He sniffed hard and wiped his face with his arm.
"I'm not crying!" he said a little too quickly, "You're crying!"
Mabel's cheeks flushed with embarrassment. She turned to clean herself up the best she could.
Dipper moved to the sled to fix it, but paused.
"Mabel," he said apologetically, "I'm sorry, for everything. Siblings…don't watch the score, they watch each other's backs."
"Sheesh," she said, "You're using Dad's lines?"
"Yeah, well," Dipper shrugged, "I'll make it up to you when this is over."
"Uh…okay," Mabel was blissfully unaware of what Dipper was keeping from her.
It's for the best, Dipper told himself, she doesn't need to know.
