Season synopsis
Contains spoilers for this season and earlier seasons.
The summer continues as Stan works to open the portal beneath the Shack, which brings back the journals’ six-fingered author, his twin Stanford “Ford” Pines. Stan recounts their childhood: he damaged Ford’s prototype, costing his brother a university opportunity and leading their family to disown him. Ford researched local anomalies and built a portal. A disastrous test prompted Ford to ask Stan to hide a journal; their later struggle sent Ford into another dimension. Stan assumed Ford’s identity, turned his cabin into a tourist trap and spent decades rebuilding the portal to bring him home. Ford offers Dipper an apprenticeship, and Mabel fears they may be separated.
Ford reveals that Bill Cipher helped him create the portal and that the unstable rift it left behind could unravel reality. As Dipper and Mabel’s birthday approaches, Bill exploits Mabel’s fear of leaving Gravity Falls: disguised as Blendin, he takes the rift from her and breaks it open. Weirdmageddon transforms the town into a demon-filled wasteland, but a barrier traps Bill’s chaos there. Dipper, Soos and Wendy enter Mabel’s fantasy bubble; Dipper persuades her to face the ruined town and leave, and they reunite with Stan and the surviving residents.
The survivors rebuild the Mystery Shack as a robot, rescue Ford and try to use a zodiac circle to defeat Bill, but the plan collapses when Stan and Ford quarrel. After Bill captures the family, the brothers secretly swap clothes so Bill enters Stan’s mind believing it is Ford’s. Ford fires the memory gun, erasing Stan’s memories and destroying Bill; the town returns to normal. Mabel’s scrapbook gradually restores Stan’s recollections. At the twins’ thirteenth-birthday party, Ford and Stan reconcile and set off to sail the world, while Stan leaves the Shack to Soos and Dipper and Mabel head home.