Amityville, Long Island, New York, 1974. Set over one year before the Lutz family moved into the dreaded haunted house located on 112 Ocean Drive. The Montelli family move into what they think would be the house of their dreams. However, after discovering that there is an underground tunnel leading into the house, an evil presence is shown to be lurking within, unknown to the family.After unusual and parnormal activities, like unknown bangings on the door, an ugly message on the youngest kids of the Montelli family's room's wall (to which their father beats them after suggesting it is them who did it and also beats their mother for intervening), the Montelli mother, Dolores (Ruda Alda) tries to have the local priest, Father Frank Adamsky (Olson), bless the house but he is driven away by her sacrilegious husband, Anthony (Burt Young), who is strict and abusive, before he can properly help. Soon afterward, the family go to church with Anthony, so he can "apologize" for being rude to Adamsky, but the Montelli's elder son, Sonny (Magner), stays as he is "not feeling well." He soon hears an alarming noise, and goes downstairs to get his father's gun, and hears laughter, following it, he then falls victim to demonic possession. Soon, a now possessed Sonny starts to have sexual feelings for his sister Patricia (Diane Franklin) and decides to "play a game" with her where he is a famous photographer and she is his model. She agrees and eventually ends up having sex with him.Patricia goes to tell Father Adamsky this, but has a breakdown while telling him; Sonny becomes more sinister and like a demon, as his face starts turning into a demon like face, startled he tries to keep his family away, but is unsuccessful due to the possession by the demon, who usually contacts Sonny through his earphones.On Sonny's 20th birthday he isolates from his birthday party, and calls Patricia who comes up to check on him. However, due to his demon phases and his body becoming more like a demon, he sends her away calling her a "damn bitch." Patricia runs away, crying, and tries to tell Adamsky that she thinks Sonny is possessed or something, but he doesn't answer. Instead, Dolores slaps Patricia after eavesdropping on them. Later that night, the evil spirit tells Sonny to "kill 'em" to which he goes and gets his father's gun, shoots his father, then his mother, his younger sister, his younger brother, and after a chase finally kills Patricia.The next day the police have arrived and pick up the bodies; Sonny is arrested, but states he does not recall of ever killing his family, and is taken away. Adamsky then realizes that Sonny is possessed and asks the church if he can perform an exorcism on Sonny but they refuse, not believing him. He therefore takes it upon himself to free Sonny, and frees him from the prison and takes him to church where Sonny escapes after seeing the crosses on the doors. Adamsky soon runs after Sonny and traces him to the house, where he performs the exorcism, releasing Sonny's soul. As the cops arrive, Adamsky asks Father Tom (Robert Pine) to take Sonny away from him, while Adamsky is revealed to be possessed. His whereabouts and what happens afterwards is unknown.