Abby and Gretchen have been BFFs 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 occurrences begin to pile up, Abby realizes there is only one possible explanation: Her best friend Gretchen, her favorite person in the world, has a demon living inside her. And Abby is not going 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?