Contains spoilers for this episode and earlier episodes.
Gravity Falls celebrates Pioneer Day by honoring its supposed founder, Nathaniel Northwest. Pacifica uses her family's connection to him to ridicule Mabel, calling her too silly to matter. Dipper finds evidence in the journal that the official founding story is false and offers Mabel a chance to expose it. She wants to prove that she can be serious, but her playful habits keep solving clues that defeat Dipper's more formal reasoning. The twins follow a folded map through the museum and cemetery while Sheriff Blubs and Deputy Durland pursue them. Stan, meanwhile, tries to avoid the celebrations and ends up locked in stocks.
The trail leads to a hidden collection of government secrets and Quentin Trembley, an erased American president preserved inside peanut brittle. Blubs and Durland reveal that they have orders to conceal the truth: Trembley founded the town, and Nathaniel received the credit to hide an embarrassing chapter of history. The officers capture the twins and prepare to ship them away with the evidence. Trembley revives, and his willingness to attempt absurd solutions helps them break free. He rewards Mabel with an official appointment and gives Dipper the president's key before leaving. Mabel decides that she no longer needs Pacifica's approval. Dipper still hands Pacifica the document exposing her ancestor, while the twins use their new key to release Stan.