15-year-old Katherine (Kat) has a dream where her father shows her the family car. It’s been horribly smashed. Kat awakens from the dream, distraught.

It’s February at a prestigious Catholic boarding school in upstate New York and the students are all preparing to be picked up by their parents for break. Kat marks off her calendar – looking forward to the day her parents will come. She’s brought to Father Brien’s office and he expresses his regret that he won’t be able to stay for the last day before break, when Kat will be playing in a recital. She becomes forlorn at the prospect of him missing her performance.

Another student, Rose, confides in her best friend that she’s worried she might be pregnant and is going to tell her boyfriend that night. When both Kat and Rose’s parents fail to pick them up before break, Rose confesses she told them the wrong date but doesn’t disclose it’s because she wants to delay them until she can tell her boyfriend about the pregnancy. Mr. Gordon, the headmaster, jokes that their parents will eventually show up since they can’t live there forever. Kat is visibly upset and the two women who are acting as chaperones, reassure her that everything’s fine and promise to keep calling her parents. They ask Rose to watch over Kat that night.

That night Rose tells Kat that she’s going out. When Kat protests, Rose tries to unnerve her by telling her a story about the nuns at the school being devil-worshipers. She leaves Kat alone and gets in the car with her boyfriend. Kat hears a payphone ringing in the hall and picks up.

Meanwhile, a girl named Joan gets off a bus in upstate New York and cleans herself up in the bus station bathroom. She has a flashback of a psych ward and tears a hospital bracelet off her wrist. She attempts to make a call on a payphone but the number she calls has been disconnected. As she waits outside for the next bus, she’s noticed by a man named Bill who offers her a ride, much to his wife, Linda’s, chagrin. Joan notices a bouquet of flowers in the back seat.

Rose is dropped off by her distraught boyfriend. She refuses to let him bring her to the clinic. She goes to the bathroom and takes a pregnancy test which she hides in the stall. She hears strange noises coming from the boiler room and goes to investigate. When she looks through a window in the door she sees Kat flailing strangely in front of the lit boiler.

Joan awakens in a hotel room. She takes a shower and a bullet scar can be seen on her shoulder. Bill knocks and tells Joan the reason he picked her up is because she reminds him of someone. He invites her to have dinner with him at the restaurant next door. She agrees.

Rose tucks Kat in and tries to reassure her that her parents are on their way. Kat tells Rose she knows her parents are dead, which disturbs Rose. Rose goes back to her room and bars her door with a chair.

Joan sits with Bill at dinner. Bill tells her the person she reminds him of is his deceased daughter and hands her a picture of Rose. Joan excuses herself and goes to the bathroom where she giggles. She has a flashback of her strangling a nurse at the psych ward named Joan and stealing her purse and her identity. Joan sees Bill talking to a cop and steals a knife from a bus tray. Bill tells her they have to hit the road to beat an impending storm and Joan goes to the car where Linda is waiting. Linda tells Joan that Bill sees Rose in every young girl but that she doesn’t see Rose in Joan at all.

Rose awakens and goes to breakfast with the two women chaperones and Kat. Kat begins to act strangely and is unable to pray. When one of the chaperones begins to scold her, Kat stands up and vomits. As they try to nurse her, she lashes out at them. Mr. Gordon rings to tell them he will be coming back to the school. The women ask Rose to shovel the driveway for him.

When Mr. Gordon arrives, he accompanied by a police officer. Upon entering the house, he grasps his face in horror.

In a flashback, Kat is seen answering the payphone and hears a gravely voice on the other end, which she calls “dad”. It asks her if she wants to stay with him. Kat is seen stabbing the two chaperones to death. Rose goes to the bathroom stall to look at the results of her pregnancy test and sighs in relief. She hears someone enter the bathroom but then leave. When she goes to the stairwell she sees a bloody body wrapped in a sheet and hurries back to the hall. Kat appears and stabs Rose to death.

The police officer finds Kat in the boiler room, kneeling in front of the lit boiler and surrounded by the three decapitated heads of her victims. She stands and begins to yell “hail Satan”. When she fails to drop the knife the cop shoots her.

Joan is in the car with Bill and Linda. Linda reveals to Joan that Rose was brutally murdered nine years earlier and decapitated. Joan says she’s going to be sick and Bill pulls over. Joan then slashes Bill’s throat and stabs Linda to death. She cuts off their heads and puts them in a suitcase which she carries to the boarding school, now all boarded up.

Father Brien visits Kat in the psych ward and performs an exorcism. When the demon leaves Kat’s body she asks it to stay with her, but it disappears.

Joan, now revealed to be an older Kat, enters the boiler room with the suitcase only to find the boiler dark and cold. She exits the school and looks back at it as she begins to sob.