Contains spoilers for this episode and earlier episodes.
The Mystery Shack prepares a reopening party after Gideon's defeat, while Stan secretly continues work on the machine beneath it. Government agents Powers and Trigger arrive to investigate unusual activity in Gravity Falls. Dipper welcomes the possibility of professional help with the journal, but Stan dismisses its contents and sends the agents away. Mabel wants everyone to enjoy karaoke rather than another mystery hunt. Dipper ignores the warning, retrieves the agents' contact information from Stan's room and invites them back. When they also doubt his claims, he reads a spell aloud to prove that the book is real. It raises a zombie, followed by an entire horde.
The undead overrun the party, and Soos becomes one of them while trying to help. Stan fights through the attackers to rescue the twins, then admits that he has always known about the town's supernatural dangers. His denials were an attempt to keep them safe. A black light reveals invisible writing in the journal, including a way to destroy zombies with a perfect three-part harmony. Mabel persuades Stan and Dipper to sing with her, turning the abandoned karaoke setup into their defense. The song shatters the zombies, and the family finds instructions to restore Soos. Stan lets Dipper keep the journal for protection but promises that he has no further secrets. The surviving agents decide their investigation must expand.