15-year-old Marnie has just spent 2 years with her grandmother Aggie. While hosting a wikt:mortality neighborhood Halloween party at their house, Marnie tries to impress a cute new boy, Kal, by showing him Aggie's magically-hidden room. Soon Aggie notices unwelcome magic (paranormal) symptoms. She and Marnie travel to Halloweentown to investigate and to fix the problem before the portal between Halloweentown and the mortal world, open only on Halloween, closes at midnight.