"Polly Peachpit." Those were the words 10-year-old Brinke Calle said when she was found covered in her own blood by the woods in rural Cannon Falls. Her best friend, Melba, had just attempted to murder her because a spider girl named Polly Peachpit told her to. Since that day, Melba has spent seven years in a mental health facility. Tomorrow is her 18th birthday. Tomorrow she'll be transferred to a federal prison. Tomorrow her real sentence will begin.
That is, until she receives a visit from FBI Agent Virgil Crockett. Crockett explains that there is another world beyond ours, where hungry, spectral aliens stalk the minds of the impressionable and weak. These things, called IMPs (Interdimensional Mental Parasites) feed on compliance. They convince hosts to do things for them, and the more they feed, the stronger they become. More IMPs stream into the world each day, invisible to everyone but his or her hosts.