Abby and Gretchen have been best friends since fifth grade, when they bonded over a shared love of E.T., roller-skating parties, and scratch-and-sniff stickers. But when they arrive at high school, things change. Gretchen begins to act . . . different. And as the strange coincidences and bizarre behavior start to pile up, Abby realizes theres only one possible explanation: Gretchen, her favorite person in the world, has a demon living inside her. And Abby is not about to let anyone or anything come between her and her best friend. With help from some unlikely allies, Abby embarks on a quest to save Gretchen. But is their friendship powerful enough to beat the devil?