During a college costume party, Peter prepares to initiate four new pledges into Alpha Sigma Rho. The four consist of Jeff, a boy from an opulent upbringing, Marti, an intelligent girl from a poor background, Denise, a promiscuous, heavy drinking party-girl, and Seth, a stoner and surfer from California. The group are forced to spend the night in Garth Manor, an abandoned mansion once owned by a man named Ramon Garth, who murdered his wife and three deformed children; following the murders, Garth hanged himself. While Garth had a fourth child, Andrew, his body was never found and legend states that Andrew still lurks within the mansion.
Peter and the rest of the students drive the four pledges up the mansion, leaving them alone at the gates. Seth and Denise leave Marti and Jeff alone to go have sex. The latter two discuss their contrasting backgrounds, before the windows in the parlor suddenly burst open and an apparition frightens Marti. In reality, Peter, along with two other students: May and Scott, have set up scares all over the mansion to frighten the pledges. While walking around the side of the house, May is pulled down a hole, where she is murdered by an unseen assailant, who violently decapitates her. Following this, Scott is also murdered by the same figure on the roof. Peter attempts to prank Denise, but she is oblivious to his efforts. He goes to search for Scott, only to discover his body strung up on the roof. He flees and attempts to escape through the fence with his key, only to be attacked by an unknown assailant. He runs into a nearby hedge maze where he discovers a second assailant, who murders him with a scythe.
In the house, the four pledges quickly discover the tricks and pranks set up around the mansion for them. Seth and Denise return to the bedroom to have sex and consume drugs; Seth leaves Denise alone to use the restroom, only to return and discover May’s severed head under the sheets. Panicked, Seth jumps the mansion gates to alert the police. Marti and Jeff also discover Scott’s body, before Marti locks herself in one of the bedrooms, while Jeff goes to investigate a light in the hedge maze. He enters the maze, where he finds Peter’s remains. He flees back to the house to inform Marti of the murder, and the pair theorize that Andrew Garth could be behind the murders.
While their backs are turned, a large figure begins to emerge from the floor behind them. Armed with a pitchfork, Jeff wounds the assailant, who seemingly disappears. They remove the rug, only to discover a trap door through which the assailant has fled. The couple descends into the tunnels below, in which they discover Denise’s corpse, along with the preserved remains of Garth’s family members.
Suddenly, a large and disfigured man appears and pursues them. Jeff attempts to subdue the man, who knocks him down a flight of steps, badly injuring him. Another killer appears, surprising the couple. The Garth brothers corner Marti and Jeff, but they are able to escape through a concealed door, fleeing back to the bedroom.
Seth finally arrives at the local police station, begging for help. The police do not take his claims seriously, believing him to be drunk, and ask him to leave. Seth pretends to leave the police station, only to take a shotgun and some bullets, before escaping through a window. He hijacks a vehicle from a civilian, informing him that he is going to Garth Manor. Seth arrives back at the mansion, where he is ambushed by one of the Garth brothers; they struggle, before Seth shoots and kills him. Alerted by the noise, Jeff and Marti meet him in the entryway of the mansion, where the other killer appears and attacks Seth, dragging him into an unlit corner of the room. Marti and Jeff are frightened by gunfire and Marti attempts to recover the shotgun. Andrew Garth emerges from the darkness and pursues Marti and Jeff through the house and back to the bedroom, where they barricade the door. Jeff urges Marti to escape out a window. Before he can follow suit, Andrew breaks through the door and hurls him to the ground below, killing him.
A frightened Marti enters the hedge maze, where she finds Peter’s corpse. She pries the keys from his fingers, before escaping. She is able to get through the gates using the keys, before attempting to escape in Seth’s stolen vehicle. She successfully hot-wires the vehicle, but is ambushed by Andrew who jumps on the roof of the vehicle. He smashes the windshield and a struggle ensues, resulting in one of the spiked gates being knocked over. Marti sees the gate and accelerates, impaling and killing Andrew. Marti loses consciousness.
Waking in the morning as the sun rises over the mansion, Marti emerges from the car with Andrew still impaled, and walks away.