Contains spoilers for this episode and earlier episodes.
A visit from Bill in Ford's dreams makes protecting the family urgent. Ford needs unicorn hair for a barrier around the Shack, so Mabel heads into the forest with Wendy, Candy and Grenda. The unicorn Celestabellabethabelle refuses to help unless Mabel proves that she is pure of heart. Mabel performs good deeds, only to be rejected again for supposedly selfish motives. While she questions whether she is a good person, Wendy and the others look for a less polite solution. At the Shack, Ford uses a machine to protect Dipper's thoughts, but avoids explaining his own past with Bill. Dipper secretly looks into Ford's memories and finds a handshake between them.
Dipper fears that Bill might be controlling Ford and threatens him with the memory gun. Ford calms him down and finally admits that he once trusted Bill as a helpful muse. The demon used flattery and knowledge to persuade him to build a gateway, intending to invade their world. Ford stopped the project after learning the truth, but the contained rift could reopen that danger. In the forest, other unicorns expose the purity test as a scam. Mabel stops accepting Celestabellabethabelle's judgments and joins her friends in a fight. They return with hair and treasure, allowing Ford to shield the Shack. Ford and Dipper promise greater honesty between themselves, while Mabel receives his thanks for protecting the family.