The film begins with a girl screaming in terror as she gazes up at the camera. During a wild college party, Peter Bennett, the president of the Alpha Sigma Rho fraternity, initiates four pledges—Marti (played by Linda Blair), Jeff, Seth, and Denise—tasking them with spending the night at the infamous Garth Manor on Hell Night. Peter, alongside his friends Scott and May, gathers everyone to recount the manor's dark history. He reveals a gripping tale: Raymond Garth, the owner and descendant of the original gold prospector, tragically murdered his wife Lillian and their three deformed children before hanging himself. The only child whose body was never found was 14-year-old Andrew, who many believe witnessed the horrific events.
Once inside the house, the pledges settle in, unaware of the lurking danger. Peter, Scott, and May attempt to scare them using prank devices. However, the night turns deadly as the pledges are picked off one by one by mysterious attackers. May meets a gruesome end, violently dragged away and decapitated, while Scott suffers a broken neck at the hands of a massive assailant. As Peter runs to escape, he discovers Scott's body dangling from a wire, only to face his own demise when he is impaled in a maze-like garden.
While chaos unfolds, Marti, Jeff, Seth, and Denise discover the setup of phony scares throughout the home. After a romantic tryst between Seth and Denise, terror strikes when a large, grotesque hand silences her. Upon discovering May's severed head, a panic ensues, leading Seth to attempt a daring escape. After injuring himself on the iron gate, he painfully scales the fence but promises to return. Meanwhile, he finds no help at the fraternity house and desperately pounds on doors to no avail.
As the horror escalates, Jeff and Marti find Scott's hanging corpse and realize they must face the terror of Andrew Garth. Armed and anxious, they lie in wait but are attacked by a figure, leading to a struggle that ends with Jeff injuring himself in a fight down a flight of stairs. Their escape is fraught with danger as they uncover a hidden lair filled with lifeless bodies. They stumble upon Denise's fresh corpse, heightening their anxiety.
Seeking assistance, Seth rushes to a police station but is dismissed as a prankster. Frustrated, he steals a shotgun and heads back to Garth Manor, only to confront a shadowy figure and engage in a deadly confrontation. After shooting the bearded man—believed to be Morris Garth, Andrew's older brother—Seth enters the manor, calling out for Jeff and Marti, blissfully unaware of the lurking threat.
As the terror crescendos, an arm suddenly seizes him, and a gunshot echoes, leaving only the abandoned shotgun rolling on the floor. Determined, Marti tries to retrieve the weapon just as she becomes the target of the other killer. The chase leads her upstairs, where she witnesses the horrifying murder of Jeff, who is thrown from a window.
In a desperate fight for survival, Marti discovers Peter's body and manages to pry the gate keys from his lifeless grip. She escapes through the garden labyrinth, hotwiring a car left by Seth. However, their relentless pursuer leaps onto her vehicle, causing a chaotic struggle. As Marti hurtles toward the gate, she rams it, impaling the killer and ending the conflict.
As dawn breaks, Marti awakens, seeming to be the sole survivor from that terror-filled night. In a daze, she stumbles away from the car, leaving behind the horrors of Garth Manor as the credits roll, suggesting that the nightmare might finally be over.
