Plot
The Montellis, an Italian American family headed by Anthony Montelli and his wife, Dolores, move into 112 Ocean Avenue in Amityville, New York, with their 4 children, Sonny, Patricia, Mark, and Jan.
An evil presence is shown to be lurking within the house, unknown to the family. Unusual and paranormal activities occur, such as unknown forces banging on the door at night when no one is outside, and an ugly demonic message painted on the wall of Jan's room. For the latter, Anthony blames and beats his children, Jan and Mark, then beats Dolores for intervening, resulting in a fight between the entire family. Concerned by these developments, Dolores tries to have the local Catholic priest, Father Frank Adamsky, bless the house. However, an argument breaks out in the family shortly after another demonic incident causes significant damage to the kitchen. Anthony blames the younger children again and hits Jan as Adamsky tries to intervene, but Anthony rudely orders him to leave. To Dolores' mortification, Adamsky leaves, disgusted at Anthony's behavior. He finds his car door open and the Liturgy of the Hours on the passenger seat torn to pieces. The situation inside the home continues to deteriorate; Anthony is shown to be strict, abusive, sacrilegious towards the Catholic faith, violent towards his family, and forcing his wife to have sex with him against her will. Dolores tries to keep things together for the youngest children.
The family goes to church with Anthony so he can apologize for being rude to Adamsky, but he only agrees after Dolores threatens to divorce him. Sonny stays at home, claiming to feel unwell. He soon hears an alarming noise and goes downstairs to get his father's gun. He hears demonic laughter and follows it to a tunnel in the basement. The unseen presence pursues a frightened Sonny to his room, and he then falls victim to demonic possession. Now possessed, Sonny approaches Patricia to play a game with him. They pretend he is a famous photographer and she is his nude model. Patricia agrees to pose naked, and the pair end up having incestuous sex. Patricia is then seen at confession, partially confessing to the act and telling Adamsky that he does it to "hurt God," but not revealing that it was her brother she slept with.
Sonny becomes more sinister and demonic as his face starts contorting demonically. Startled, he tries to keep his family away but is unsuccessful due to the demon's influence. On Sonny's birthday, he isolates himself from the party, but Patricia goes to check on him. She tells him she does not feel guilty about what they have been doing, but due to his demonic phases and his body's gradual demonic contortions, Sonny sends her away, using foul language. Patricia runs away crying and tries to tell Adamsky that she thinks Sonny is possessed, but he does not respond. Later that night, the evil spirit tells Sonny to "kill them all." Sonny, now fully possessed, goes and gets Anthony's rifle and kills him, Dolores, Jan, Mark, and finally Patricia, after hunting her down.
The next day, the police arrive, pick up the bodies, and Sonny is arrested. Sonny tells Adamsky at the scene that he does not recall killing his family. Adamsky then realizes that Sonny is possessed and asks the bishops if he can perform an exorcism on him, but they refuse, not believing him. He then decides to perform an exorcism without the support of the Catholic Church. After freeing him from police custody, Adamsky takes Sonny to church. Sonny attacks him and escapes after seeing the crosses on the doors. Adamsky runs after Sonny, traces him to the house, and performs the exorcism, releasing Sonny's soul. As the police arrive, Adamsky asks Father Tom to take Sonny away from him. Tom takes Sonny outside, where the police arrest him and take him back into custody. It is revealed that the demon has entered Adamsky. Father Adamsky's fate, he was never seen or heard from again, and the house is eventually put up for sale, destined to be purchased by the Lutz family.
