In February, at a prestigious Catholic boarding school in upstate New York, students are preparing to be picked up by their parents for break. Kat (Kiernan Shipka), a freshman, has a dream where her father shows her the horribly smashed family car. She awakens from the dream, and marks off her calendar, looking forward to the day her parents will come. An older student, Rose (Lucy Boynton), worries she might be pregnant and intends 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 nuns who are acting as chaperons 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 (Emma Roberts) 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 (James Remar) who offers her a ride, much to the chagrin of his wife, Linda (Lauren Holly). Joan notices a bouquet of flowers in the backseat.
Once dropped back to the school by her boyfriend, Rose goes to the bathroom and hears strange noises coming from the boiler room and goes to investigate. When she looks through a window in the door she sees Kat prostrating 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. In a flashback, a police officer shoots a rifle. 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, where he shows Joan a picture of Rose, who is revealed to be his daughter. He explains she died nine years ago. Joan excuses herself and goes to the bathroom where she giggles and has a memory of killing a woman and stealing her identification card. 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 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. Rose awakens and goes to breakfast with the two chaperons and Kat. Kat begins to act strangely and is unable to pray. When one of the chaperons 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. It is implied that Kat's parents have indeed died in a car crash. The women ask Rose to shovel the driveway for him. After shoveling, Rose tries to re-enter the chaperons' house but the doors are locked, so she returns to her dorm room.
When Mr. Gordon arrives at the house, he is accompanied by a police officer. Upon entering the house, Mr. Gordon 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 tells her that her parents aren't coming, and to kill everyone. Later, Kat is seen stabbing the two chaperons to death. While one of the victims crawls along the floor, mortally wounded, Kat casually walks upstairs and returns with two pillowcases. She then finishes the job. Rose goes to the bathroom and gets her period, sighing in relief. She hears someone enter the bathroom but then leave. When she goes to the dorm stairwell she sees two bloody heads wrapped in pillowcases and hurries back to the hall. Kat appears and stabs Rose to death before decapitating her.
The police officer finds Kat in the boiler room, kneeling in front of the lit boiler and surrounded by the heads of her three victims. She stands and begins to yell "Hail Satan!" When she fails to drop the knife, the cop shoots her in the shoulder.
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, now revealed to be a younger "Joan", in the psych ward and performs an exorcism. Kat sees a shadowed, demonic figure; she asks it to stay with her, but it disappears. In present day, Joan 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, realizing she is truly alone.
Storyline
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. — InternetGhost