Mabel knew something was wrong about her brother ever since they liberated Bill. He didn't act the same at all. No, he had turned into a masochistic, sadistic lunatic. The kid went around stabbing forks into his arms for fun.
Her Grunkles dismissed his behavior as that of a teenager's. They punished the twins severely for sneaking out, and made them promise to never visit the cave again. They did, wholeheartedly. Well, at least, Dipper did.
Mabel went back to the opening of the cave and found a large rock in place, blocking it off. She tried, but was simply not strong enough to lift it. The boulder was immobile, neither her nor Wendy could lift it.
The girl decided to relax. Bill was gone, she told herself. Everything was fine! And they didn't have to sneak out anymore. That was good, too, wasn't it? Yes, everything was quite fine.
Until she woke up. "Good morning, world!" she cheerfully chirped, and glanced over to Dipper's bed. "Good morning, Dipper!"
His form laid in the bed, unmoving. Which was better than what she could say for some nights. She had found him drunk before, self harming and asking why it didn't hurt. She ignored the memory, as she jumped out of bed and ran onto Dipper's, and began jumping. "WAKE UP, WAKE UP, WAKE UP!" she cheered, grinning, until the sheets parted and Dipper was not there, only pillows to mimick his form.
"Dipper...?" she asked, feeling foolish. Why wasn't he here? Where did he go? Quickly, she searched every inch of the attic, and then the main floor. Nothing. Which was extra odd. He had never done this before, never disappeared without a trace, and never left something to pretend he was there.
Mabel went to her Grunkles' bedroom. The brothers shared a room, but slept in separate beds, much like Dipper and Mabel.
"Guys! Dipper's missing!" she yelled.
Stan woke up almost immediately. "What? What do you mean, missing?"
"I mean, I woke up, and he wasn't in his bed!" she said, fear in her eyes.
Ford grunted, waking up. "It's too early for this..." he muttered, donning his glasses.
Stan ignored his brother. "Are you sure he isn't here, Mabel?"
She nodded, and ran off to check the house once more.
When she returned, the twins were fully dressed and ready to go. They went about the house, calling his name, for a while, until Stan said, "Did you check the roof?"
She frowned, and shook her head, and rushed off to do so. And when she did, she was met with a disturbing sight - the sky was red, and a giant X crossed it, blotting out where the sun should be. Her quavering, panicked voice screamed, "DIPPER! WHERE ARE YOU?" but there was no answer, but for the echoes and unsettling silence.