When Gravity Falls' holiday Pioneer Day comes around, Grunkle Stan, Dipper, and Mabel stroll through town, looking at many various out-dated activities and ceremonies, Grunkle Stan really hates Pioneer Day and tries to leave, but fails. Later, Sheriff Blubs calls the town to a meeting where Pacifica Northwest makes an announcement about her great-grandfather, Nathaniel Northwest. Pacifica asks for volunteers to say something about themselves. Mabel comes up, but Pacifica makes Mabel feel bad by telling her she's too silly, and Mabel decides she has to be more serious by taking off her nacho earrings and her sweater and putting her serious face on.
Dipper then finds an article in his journal that says Nathaniel Northwest may have been a town fraud. Mabel decides that solving a mystery will prove that she can be serious. Enclosed was a map with the triangle symbol. Behind a statue, Sheriff Blubs and Deputy Durland hear Dipper and Mabel's plan. He tells an anonymous figure with several cameras.
Dipper and Mabel head to the library to look up the symbols on the map. Dipper finds a symbol that means fire and is the same shape as the picture on the documents. Dipper thinks they should burn the document, until Mabel makes a hat out of the paper, which shows them a map to the Gravity Falls Museum of History.Dipper dramatically says they're going to have to break in! Dipper and Mabel head to the Museum where a lady says "Here are your free Pioneer Day passes and balloons, blue and pink!" Dipper then dramatically says, "We're in!" There's an abstract painting shaped in a triangle, and when Mabel turns upside down, it reveals a pointing 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, and Dipper says to watch for booby traps. Mabel then laughs at booby traps. They go down the hallway when Mabel steps on a button, which triggers tranquilizer darts. They dodge the darts and go down a hallway revealing many top secret documents (e.g., Abraham Lincoln's hat was covering up a hand on his head; Ben Franklin secretly was a woman).
They come across a document called the Northwest cover-up, and it is revealed that the true founder of Gravity Falls was Quentin Trembley. Sheriff Blubs and Deputy Durland reveal that they have been following the twins and had been tasked with finding Trembley, playing a film for Mabel and Dipper showing the truth about Gravity Falls. Sheriff Blubs then says he must ship them off to Washington D.C., revealing that Quentin Trembley had encased himself in a peanut brittle cube in the hopes it would keep him alive.
Sheriff Blubs and Deputy Durland get on train bound for Washington D.C., having placed Dipper and Mabel in a giant crate along with the still encased Trembley. Mabel, in sadness thinking that she caused this, eats part of the peanut brittle, freeing Trembley, who miraculously survived as a result of the peanut brittle's "life-sustaining properties" (as said by Mabel). They escape, but are caught by Blubs and Durland. They climb to the roof, where Trembley recalls that he never signed a resignation, and that he ate a salamander and jumped out the window instead. Trembley then orders the cops to stop and take a vacation.
When Dipper, Mabel, and Trembley head back to town, Trembley decides to make Mabel a Congressman and gives her a hat. Dipper gets the President's Key, which can open any lock in America. Back in town, Mabel accepts her silliness and doesn't care if Pacifica knows the truth or not, but Dipper shoves the proof in Pacifica's face, calling her family a 'sham' and telling her to "deal with it." He then says that revenge is underrated and that it felt good.
During the end credits, it shows Quentin Trembley and the babies of the Supreme Court. Trembly is trying to discuss things but it's hard with four babies, and Quentin comments it is a "dark day for America."
- Story by:
- David Slack
- Written by:
- Directed by:
- Storyboards by:
- Mark Garcia
- Chris Sonnenburg
- Niki Yang
- With the Voice Talents of:
- Additional Voices:
- Casting By:
- Sara Goldberg
- Dipper ends up accepting Soos' nickname for Dipper and Mabel, the 'Mystery Twins,' from "Headhunters."
- Mabel learns to accept her silliness in this episode.
- This is the first episode that Soos does not appear in. However, Grunkle Stan mentioned that he came by and, "Talked to him for like an hour."
- Viewership: This episode was watched by 3.869 million viewers on its premiere night.
- Gideon makes an appearance dressed, as he put it, "a humble tomato farmer" and that he was "selling his wares".
- "Dysentary" is a sickness that spreads through water.
- Part of the secret documents read, "A time devouring baby is trapped in a glacier in Antarctica." This is probably a reference to the Time Baby who appears in the next episode.
- Bill appears on the negative twelve dollar bill (as a reference to the Eye of Providence).
- The cave the Cover-up is hidden in has what appears to be a cracked Liberty Bell, which in real life had to be recast twice.
- On the additional paragraph on the secret of The Northwest Cover-up document reads:
- Thomas Jefferson was actually just two kids in an overcoat standing on each other's shoulders.
- The current and forever President of the United States is actually Santa Claus. Under the reign of Mr. Claus, America is not a democracy, but a jollyocracy.
- The statues in Mount Rushmore are actually gigantic presidential-faced robots that will be called into action when America needs them most.
- An enormous, evil, time-devouring baby from another dimension is frozen in an Antarctic glacier. Fortunately glaciers never melt, so we should be fine.
- Writing jokes for cartoons is more important than sleep.
- If you recite the pledge of allegiance backwards, you'll gain secret wizard powers! (This one is true, kids! Try it at home!)
- During the ending credits of this episode, there is a cryptogram that reads "V. KOFIRYFH GIVNYOVB." Once decoded, it reads "E. PLURIBUS TREMBLEY." In Latin, this means "Trembley out of many."
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