It was just an innocent letter. She didn't think their parents would come barging through the front door of the Mystery Shack to drag them back home.
"No, please," she begged, trailing after them as they shoved their way past Grunkle Stan so they could start tossing her and Dipper's clothes into their suitcases like they could't get their kids away from the man that claimed to be someone he wasn't fast enough, "this is all my fault. You can't just take us away like this. He didn't do anything-"
"He lied to us, Mabel," her father snapped at her, concern, frustration, and hurt making him lash out now that he knew his kids were safe, "for decades. He lied about his name, he faked his own death…" Paling, he muttered almost to himself, "I left my children in the care of a con artist."
When he faltered, his wife paused in their packing so she could lay a comforting hand on his arm. "We both did. But they're safe. They're safe." Mabel listened to her repeat the words until some of the color returned to her father's face, her head spinning because the words were spoken like it was some sort of miracle, but Grunkle Stan would never hurt them. Never.
It didn't matter, though, because her parents weren't listening. On her way out, she noticed Dipper standing frozen in the hallway, confusion melting into anger that contorted his youthful features into something ugly in the dim lighting of the afternoon. "Mabel," he whispered, so low only she could hear, "what did you do?"
She shook her head helplessly, tears already running down her cheeks. After this, Dipper might never speak to her again. She was going to lose Soos and Grunkle Stan and Grenda and Candy and Wendy and Grunkle Ford. There was no way she could lose Dipper, too.
Glancing over her shoulder to see her parents snapping their suitcases shut, she said with an odd, ghosting twitch of her lips, "Uh, Mom, Dad, can I show you something amazing before we go? Grunkle Ford built it himself!" Be enthusiastic, she reminded herself. Be convincing.
Her parents exchanged meaningful glances before her mom hedged, "I don't know, Mabel. Your father and I would much rather leave as quickly as possible." Behind her, their father looked thunderous, having glimpsed Stan still standing on the bottom floor of the Mystery Shack, as though his feet were rooted to the ground.
Making her eyes as wide and shiny as she could, which wasn't hard given the circumstances, Mabel said, "If you just do this one thing, I swear Dipper and I will leave without a fuss. Isn't that right, Dipper?" Dipper let out a squawk of protest. With a nervous giggle and a sidelong glance at her twin, Mabel translated for him, "He says yes."
And since a quick getaway required the cooperation of both their children, their parents reluctantly agreed. When Dipper moved to follow, brow furrowed so low it practically touch his nose, Mabel shook her head. Were it any other time, Dipper might have ignored her and followed, anyway, but tears still spilled unchecked and her smile hadn't lost its unnatural jerkiness. More than anything, he wanted to yell that she'd ruined everything, that Grunkle Stan was probably going to be arrested and it was all her fault, but he couldn't bring himself to do it when she looked at him like that. "It's okay, Dipper," she whispered before following their parents down to the basement. "I'm going to fix this."
The secret entrance had her father almost trembling with rage. "Another secret?!" He yelled to no one in particular. In some ways, he was taking things worse than Dipper had, but that was because he'd known Stan his entire life, ever since he could remember the man had helped him, been there for him, from his first steps to his college graduation. Finding out he'd been deceived all that time, that the man he'd looked up to was a liar and thief, stung like battery acid in a open wound.
And Mabel did the best she could. In the peppiest voice she could muster, she showed off the portal as she described all the wonderful, incredible, amazing things she'd seen in Gravity Falls: gnomes, zombies, an evil triangle named Bill that possessed Dipper once.
To her dismay, the attempts to make her parents understand that Gravity Falls was an exciting place, filled with adventure and mysteries, only seemed to make her parents more determined to take her and Dipper away. "Stan let you two play in the woods by yourselves?" Her fathered accused, disbelief coloring his tone. "What if something had happened to you? What if you'd been hurt?!"
More fears and buried rage poured out into the open, with her father swearing he'd see their grunkle behind bars, just as Dipper had suspected he would. When he moved to climb back up the stairs, Mabel leapt to follow, only to stop when light unexpectedly shined in her eyes, temporarily blinding her. She blinked at it, tilting her head until she could clearly see the source, light refracting off a glass bulb.
When her father turned around to see why he wasn't hearing any footsteps following him up the stairs, it was to see his daughter pointing a gun-shaped contraption at his head. "Ma-"
Something white, so bright he had to shut his eyes, came rushing towards him moments after her finger pressed the trigger, and he sank, limp and lifeless, onto the stairs.
Seeing her husband fall, Mabel's mother started screaming, high-pitched and shrill, and Mabel - stressed, gasping, frightened - shot her too, barely even thinking. She just wanted the sound to go away.
Having heard the screams, Grunkle Stan, Ford, and Dipper came bursting downstairs. "Mabel, sweetie, are you alright?!" She wasn't. She was sobbing, broken on the floor. Stan, seeing his brother's son lying on the stairs, grabbed him by the collar, and threatened, "I don't care if you are Shermy's son. You make that girl cry again and I'll-" The man's head lolled unexpectedly to the side, his mouth parted slightly in a familiar guileless expression.
That was when they noticed the object clutched in Mabel's trembling, white-knuckled hands. It wasn't hard to draw the right conclusions after that.
"Hey, sweetie," Stan kneeled at her side, keeping her distracted as Dipper gently tugged the Memory Gun from her fingers so he could toss it aside, "it's okay. I know why you did what you did. And I'm not saying what you did was right, but I'm grateful. One summer just isn't enough time to spent with you crazy kids." She leaned against him, weeping into his chest, clutching his shirt in her little fists, and while she cried herself out, he stroked her hair, one steadying hand pressed against her back to remind her that whatever happened, whatever the consequences, she wasn't going to be facing them alone.
Once their parents woke up, Stan explained to them that they'd come down to the Mystery Shack to visit, but hit their heads falling down the stairs shortly after arriving. "Amnesia, am I right? It's the worst." Ford wanted to tell them the truth about who he was, about where he'd been, but conceded that it would have to wait. They tried to spin a tale where Stanford was instead the presumed dead Stanley, who had returned after disappearing for thirty years due to a separate, more severe bout of amnesia. "Amnesia!" Stan crowed. "It's the answer to all of life's problems. Just ask McGucket!" Wincing, he added, "On second thought, forget I said that."
While Ford tried to create a more plausible reason for their memory loss, Dipper wandered up to the attic to find the door closed. "Mabel, come on, let me in."
"Go away, Dipper." There wasn't any bite or emphasis to the words; as though she didn't want him to leave so much as she didn't care, which was maybe even more worrying, because Mabel cared about everything. Except unicorns. But that was a recent development.
Sighing, Dipper pulled out a lock picking kit Grunkle Ford had given him from his back pocket and went to work. After a few minutes of tinkering, the gears inside the lock snapped open with a click, the door swung open. Tentatively, he peered inside. "Mabel?" He'd fully expected to find her curled up in the corner, her sweater bunched around her face so only her hair stuck out, but that wasn't the sight waiting for him in the room they shared.
Shreds of paper clung to the walls, the last remnants of his sister's posters. Her sweater lay in a crumpled heap on the ground, even though Mabel loved her sweaters, every single one, and always took the time to fold or hang them up when she changed out of them. She was staring at the ceiling when he approached. At first, he wasn't sure she knew he was there, but when he cleared his throat to speak, she said, "I did something terrible, Dipper." And now that he was close, Dipper could see she was trembling. "But I didn't know what else to do." And since he didn't know what he could say to banish the plaintive tone in her voice, to make the helplessness dissipate into something lighter, more bearable, he sat on the edge of the bed and wrapped his arms around her.
"I noticed your sweater was on the floor," he muttered once he noticed the goosebumps on her skin. When his sister didn't respond beyond a miserable sniffle, he twisted his head to meet her eyes, something small and genuine tugging at his lips. "Mind if I be your sweater for a little while?"
Outside their door, Ford and Stan listened to the sounds of Dipper's low murmurings and a watery giggle. They'd come up to check on them, since their parents were all settled, sleeping off the worst of the Memory Guns side effects in the living room downstairs, but it seemed their presence wasn't required at the moment. Dipper was helping the healing process along in ways they'd dreamt of but never fully realized. It was an odd feeling, a mix of pride and melancholy that accompanied the realization that their twelve-year-old nephew was better at being a brother than they'd ever allowed themselves to be.
"Do you think," Ford turned to look at Stan, his expression grave, "those two would be better off if they never met the two of us?"
And Stan wasn't sure what he wanted him to say, so he told him the only truth he knew for certain. "I've met a lot of people, Ford. Meeting me's the worst thing to happen to most of them."