Gideon has become distrustful of Pacifica after witnessing her and Dipper actually being on good terms. Gideon hates Dipper's guts and believes him to be one of Mabel's oppressors. Feeling angry and alone, he runs into his dead sister's ex-boyfriend, Robbie, who dislikes the Northwest just as much as he does. He teaches Gideon how to summon Bill, who offers the desperate boy the power to save Mabel at the price of a favor at a time of Bill's choosing. Gideon accepts.
The power Bill infuses Gideon with is from the same cosmic void where Bill was created, and it corrupts him, making his innocent goal to rescue Mabel an obsession and the idea of hurting people, especially Dipper, suddenly very appealing. He attacks Northwest Manor and kidnaps Mabel, and when Stanford refuses to do anything to get her back, Dipper rushes off to find Pacifica and her friends for help. Even though he's convinced that Gideon's just doing a bunch of special effects tricks and is wearing some weird costume, Dipper knows that something is very wrong and his sister is in danger with him. Dipper begs Pacifica and the girls for their help. They agree, and the four race off to find Gideon and Mabel.
With Mabel "saved", Gideon has turned his wrath onto the townspeople for letting the Northwests sweep Mabel's existence under the rug as Mabel tries to reason with him. Pacifica and co. lure Gideon to the Pines Bed and Breakfast where she has a bunch of magic objects that she thinks can save Gideon from his corruption. As the big showdown goes down, Mabel escapes Gideon's grasp, and Dipper immediately grabs her and drags her away as fast as possible, ditching the girls.
Pacifica is furious, but they still manage to win the fight and save Gideon. He sheepishly explains what happened, leaving out the part where he still as his part of the deal with Bill to hold up. Unbeknownst to him, a small part of his powers still remain inside of him, left over. The Bed and Breakfast has been destroyed as a result of the fight, and Dipper begins to turn away from Ford.