Sheriff Joanna Dawson’s quiet life in Shelby, Montana, shatters when her son’s best friend, Holden, is found brutally murdered. The crime bears the chilling signature of the "I-15 Killer," a serial murderer who leaves taunting notes signed "The Exorcist." As Joanna races to uncover clues, the FBI intervenes, revealing a twisted pattern of killings across state lines. But when a corrupt agent sabotages the investigation and bodies pile up, Joanna confronts a web of conspiracies—and a killer obsessed with purging "demons" through ritualistic violence. With her family at risk and her past in Billings haunting her, Joanna must outwit a psychopath before another life is lost. A procedural dive into small-town terror, fractured justice, and a hunt for redemption.