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.

After being challenged to a video game battle by Wendy's boyfriend Robbie, Dipper is shocked when his favorite playable fighter, Rumble McSkirmish, comes to life to defend him. But Dipper soon loses control over Rumble McSkirmish and has to find a way to stop him.