After exposing a duo of con artists alongside his partner Melanie at the notorious 112 Ocean Avenue in Amityville, Long Island, New York, recently divorced John Baxter (Tony Roberts), an employee of Reveal magazine, finds himself enticed into purchasing the infamous haunted property, guided by the persuasive Clifford Sanders. As John prepares to make the house feel like home, Clifford bravely ventures into the eerie abode alone. His investigation leads him to the attic, where he encounters a mysterious presence, causing him to become trapped in a room filled with a swarm of flies. When John arrives, he discovers Clifford, whose cause of death is believed to be a heart attack or stroke, but this does little to deter John from his new home.
Oblivious to the unsettling incident, John is further dismissed by the peculiar situation when Melanie shares a series of haunting photographs she took of Clifford, each depicting him as a decaying corpse. Despite this, John remains steadfast in his belief that the house is normal, assuring Melanie they will reunite there later. While John tends to work and grapples with odd mishaps like elevator troubles, Melanie ventures back into the house. There, she encounters a maid who soon leaves in fright. Alone in the house, Melanie descends into the basement to examine the fuse box; however, an inexplicable chill sends her scurrying back upstairs just as the basement door flies open, subjecting her to an arctic blast of wind. Later that night, John finds her in a state of sheer panic, curled up against the wall.
Melanie’s desperate attempts to sway John into recognizing the malevolent force residing in the house fall on deaf ears. Soon after, while scrutinizing enlarged versions of the eerie photos, she uncovers a demonic visage lurking within them and races to warn John. Tragically, as a result of her panic-fueled rush, she crashes her car, leading to her fiery demise as the photographs ignite into flames. Authorities, including John, rule her death an accident, with John completely unaware of the malevolent essence pervading their home.
Amid this chaos, John's daughter Susan and her friend Lisa, intrigued by the house’s sordid past—specifically its origin on an Indian burial ground—decide to hold a Ouija board session in the attic. The game abruptly halts when the board ominously warns that Susan is in danger, and a glass they were using shatters against the wall, leading Susan’s friends to think it was all a prank. Bored and restless, they venture out on John's motorboat, and while Susan’s mother, Nancy, searches the house for her daughter, Susan returns soaked but unbothered. Ignoring her mother’s calls, Susan heads straight to her room, locking the door behind her.
When John returns home with groceries, he stumbles upon chaos at the docks, discovering Susan's lifeless body while her friends claim she fell from the boat. This harrowing scene leaves Nancy devastated, especially as she tries to convince John she had just seen Susan alive in the house. In her grief, she becomes unyielding, unwilling to leave the premises for Susan's funeral.
Haunted by disturbing dreams of an old well in the basement, which is rumored to be a gateway to Hell, John consults his friend Doctor Elliot West for guidance. Elliott proposes that a team of paranormal investigators should explore the house to determine if Nancy genuinely witnessed something supernatural. John, desperate for answers, reluctantly agrees.
As the investigators settle in, Nancy has a chilling confrontation with a spectral figure that eerily mimics Susan's voice, luring her toward the enigmatic well in the basement. Determined to uncover the truth, John and Elliot attempt to engage whatever dark presence lurks in the well, believing it may be able to return Susan. But chaos ensues when Elliot is suddenly dragged into the depths by a demon, causing pandemonium as Susan’s apparition fades away. The house begins to tremble violently, launching dangerous objects throughout, which ultimately leads to the demise of most of the investigative team.
In a narrow escape, John, Nancy, and a few others flee through a window moments before the house erupts in flames, leaving behind destruction. As the remnants of the house smolder, an ominous sight catches their eye—a single fly emerges from the depths of the old well, hinting that the terror may not yet be over.
