Contains spoilers for this episode and earlier episodes.
Mabel prepares for the twins' thirteenth birthday, eager to celebrate before summer ends. Her excitement fades when Wendy describes high school as miserable and Candy and Grenda reveal that they cannot attend the party. Dipper has a different day with Ford. A crack in the rift's container sends them into a buried alien spacecraft to find an adhesive strong enough to seal it. During the expedition, Ford offers Dipper an apprenticeship that would let him stay in Gravity Falls after summer. Dipper worries about leaving Mabel but becomes more confident after rescuing Ford from a security robot. He accepts, imagining a future devoted to the mysteries he loves.
Mabel overhears the decision and sees it as losing the one person she expected to face growing up beside. She runs into the forest carrying Dipper's backpack, which contains the rift. Blendin appears and offers her a way to extend the summer, claiming that an object in the bag can power it. Unaware of the rift's danger or Ford's instructions to keep it secret, Mabel agrees to hand it over. Blendin is being possessed by Bill, who exploits her distress to obtain what he needs. He breaks the container and reveals his deception. The opening tears across the sky as strange forces enter the town, leaving Dipper and Ford to watch the catastrophe they were trying to prevent.