In Amityville, New York, the same town where Ronald DeFeo Jr. murdered his family in 1974, a Catholic priest is shot to death in a confession booth inside his parish church. After the shooting, the booth is removed and stored in the basement of the clergy house.
Twelve years later, Marvin is a psychologist who, with his wife Debbie Klein, buys the clergy house. The couple invites their three friends: Frank Winfield, Bill, and Abigail Blaine to help renovate. [Debbie Klein] is immediately perturbed by the house and hears noises from the basement, but Marvin urges her to note down her nightmares for his analysis. The next morning, the group is visited by the eccentric former church secretary, Mrs. Moriarty.
Marvin and Bill explore the basement, and Bill finds the confession booth, among other church artifacts. After an outbreak of apparent poltergeist activity, the house seems haunted to everyone except Marvin. To calm nerves, they plan to go out for dinner at a local bar. [Frank Winfield], suffering a migraine, stays at the house. At the tavern, Marvin speaks to two older men about the town's paranormal activity and dismisses it as mass hysteria. The men bring up a boy who murdered his family and possibly was possessed, implying the house Marvin is in might be possessed. Meanwhile, [Debbie Klein] encounters Mrs. Moriarty again in the bathroom, where she cryptically rants about the priest who once lived in the home.
The next day, Mrs. Moriarty stops by again but is confronted by an unseen assailant and thrown down the basement stairs to her death. Her murder is inadvertently recorded on a video camcorder Bill left in the house. Meanwhile, Debbie has a disturbing vision of a man hanging from a tree in front of the house. Police are summoned after Mrs. Moriarty's body is found, and a detective tells Marvin about her connection to the house; he also reveals that police identified the priest's killer as a local teenage boy who hanged himself before he could be caught.
Abigail is disturbed by the event and leaves. Marvin begins attempting to decipher Debbie's dream diaries, which Bill realizes consist of Latin exorcism writing. Debbie suffers a nightmare in which she witnesses the truth of the priest's murder: He was shot to death by his illegitimate son, conceived during a sexual tryst with a female parishioner who sought vengeance for being abandoned.
She wakes to find Marvin dead in the confession booth in the basement. There, she is confronted by Frank, now possessed by the priest's illegitimate son. Meanwhile, police reviewing the video tape identify Frank as the perpetrator based on the shoes he wore captured on tape.
Frank chases Debbie through the house, during which she disfigures him by burning his face. He continues to pursue her, attempting to impale her with a processional cross. Debbie manages to stop him by shooting at him with a nail gun. Abigail returns and finds Frank, apparently dead, but he reawakens and begins strangling her. Using the processional cross, Debbie stabs Frank through the chest, impaling him to death. Later, Debbie and Abigail are escorted out of the house by police. A detective shows Debbie a photograph of the infant boy—the priest's illegitimate son—found at the crime scene. He asks if it is hers, to which she responds that it belongs to the house.
