Film special effects artist Orville Kruger, [Guy Stockwell], brings his daughter Lisa [Linda Blair] and her friend Kathy [Donna Wilkes] to the family's remote country house in the San Bernardino Mountains. On the road, Lisa and Kathy are harassed by a gang of punks driving a Volkswagen van, led by Scratch [Brad Wilson], who are planning to rob the Krugers' home. Scratch and his crew are on the run after a robbery and mass murder of a family in Nevada.
Lisa and Kathy arrive at the house, where Lisa's mother informs them that Lisa's uncle Rod [Tab Hunter], a plastic surgeon, is arriving the next day. In conversation, Lisa asks how “Patrick” is doing, and Orville later plays a prank on the girls using his special effects creations. That night, the gang holds the family hostage and threatens them, demanding their valuables. When Orville states they have none, the gang bludgeons him to death with a log before killing Lisa's mother. From behind a glass slot in a wall, a pair of eyes look on as the gang brutalize the Krugers.
The gang begins a rape attempt against Kathy before a female member of the criminals stabs her to death. Lisa manages to escape the house and flees on foot. The gang proceeds to ransack the house before departing, and finds a hidden room decorated like a nursery. A hideously deformed man—revealed to be Lisa's adopted brother, Patrick—murders two of the gang members before the rest flee into the woods. Scratch and his girlfriend, Shelley [Michelle Bensoussan], decide they will fight Patrick. At dawn, Lisa is attacked by one of the gangsters, but Patrick arrives and saves her, killing him.
Rod arrives at the house and finds police there, who suspect the missing Lisa may have committed the murders. Lisa is later found unconscious in the woods by authorities. Meanwhile, Scratch and Shelley come face to face with Patrick, who attempts to kill them, but he is subsequently shot dead by police. Lisa, suffering a blood clot, is hospitalized, while Scratch and Shelley falsely claim to have been innocent victims of Patrick. Lisa subsequently dies during an emergency operation; with the only witness to the crime now dead, Scratch and Shelley are due to be released by police.
Rod tells authorities that the deformed Patrick was abandoned as a child and raised by the Krugers, who kept him contained to ensure he would not hurt anyone. Upon realizing that Scratch and Shelley are to be freed, he rents two surgical tables and procures a shotgun. Outside the jail, Rod kidnaps the two and drives them back to the Krugers' country home. In a makeshift operating room, Rod reveals that he too is deformed, removing a mask made by his brother Orville to conceal his facial deformity. Rod surgically mutilates Scratch and Shelley before locking them inside Patrick's soundproof nursery.
The events are subsequently shown not to be real, but a test screening of a feature film hosted by the Frankenstein Monster and the Wolfman. The Frankenstein Monster and the Wolfman burst into the screening room, frightening the audience.
