Two Catholic schoolgirls Kat (Kiernan Shipka) and Rose (Lucy Boynton) get left behind at their boarding school over winter break as the other girls leave, where it's rumored that the nuns are satanists. Meanwhile, a disturbed mental patient Joan (Emma Roberts), an escapee, is picked up by a middleage couple (James Remar and Lauren Holly) who drive her on a determined trip to the same school, where the girls must face the supernatural and demonic possession.