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When Dipper and Mabel discover evidence that the reported town founder is a hoax, they set out to expose the historical cover-up and prove that Mabel's silliness is not a bad thing. When Gravity Fall's holiday, Pioneer Day, comes around, Pacifica Northwest makes an announcement about her great grandfather, Nathaniel Northwest. Pacifica makes Mabel feel bad by telling her she's silly, and Mabel decides she should be more serious. Dipper then finds an article in his 3 book that says Nathaniel Northwest was a fraud. Enclosed was a map, with the triangle symbol. Dipper thinks they should burn the document, until Mabel makes a hat out of the paper, which shows them a map to Gravity Fall's Museum of History. There's an abstract painting shaped in a triangle, and when Mabel turns upside down, it reveals an angel, and she comments that she saw that statue in the cemetery. Once at the cemetery, Mabel sees the statue, and she pretends that the statue is picking her nose. She bends the statue's finger, opening a secret passageway. Mabel and Dipper enter it, revealing many Top secret documents. They come across a document called the Northwest cover-up, and the true founder of Gravity Falls was Quentin Trembley.
Source: Disney Channel