In 1935 Florida, a lowlife mobster named Nicky Rocco is betrayed and executed in the swampy backwoods as his pregnant gun moll Ruby Claire, Piper Laurie, watches. He swears vengeance with his dying breath, and then she suddenly goes into labor. Sixteen years later in 1951, Ruby is now running a drive-in theater in the backwoods near her home and employs ex-mobsters to run the place. Her 16-year-old daughter, Leslie Claire, Janit Baldwin is mute since birth, and resides in the home with Ruby, her lover and henchman Vince, Stuart Whitman, and Jake Miller, a blind, wheelchair-bound former mobster who had his eyes cut out. Ruby spends her days overseeing the theater's operations as well as lamenting her short-lived career as a lounge singer.
One evening during a drive-in showing, the projectionist Jess Littinger, Eddy Donno, experiences poltergeist activity in the projection booth and then is found hanged. Ruby dismisses his death as a suicide and instructs Vince to get rid of his body. Meanwhile, Lila June, a patron at the theater, is given a necklace from Louie, a concession stand clerk Paul Kent that he stole from Ruby. He attempts to rape Lila, but she flees. Louie is accosted by an invisible force in the woods that kills him. The following morning, Leslie and Vince find Louie's bloodied corpse hanging from a tree. Ruby fears that Nicky's spirit has returned to torment her, believing that he has found a way back to the living.
At nightfall, Vince and Ruby take Louie's corpse to the swamp to dispose of it. Later, Dr. Paul Keller, Roger Davis, a prison psychologist who helped Vince receive an early parole and who claims clairvoyance, investigates the property and tells Vince that he senses a supernatural presence stalking the drive-in. Vince and Dr. Keller arrive at the house to speak with Ruby about his findings. Later, a drunken Ruby goes to the drive-in grounds to speak to Nicky, and she is confronted by the sound of his voice calling her name over the theater's sound system, followed by a series of violent apparitions.
In the middle of the night, Ruby is confronted by Leslie in the attic, who begins speaking in Nicky's voice. A violent struggle ensues, and Ruby knocks Leslie unconscious. Dr. Keller attempts to hypnotize Leslie after the incident. During the attempt, Ruby watches in horror as bullet holes appear in Leslie's head, the same as those that inflicted Nicky. Later, Dr. Keller and Ruby discuss Nicky's death, and while Ruby admits to having set up other mobsters, she was not responsible for Nicky's death. Suddenly, Jake appears in his wheelchair, stabbed to death. Ruby and Dr. Keller rush upstairs to Leslie's room and find her body contorted and her speaking in Nicky's voice.
Ruby goes downstairs while Dr. Keller attempts to calm Leslie and attempts to speak with Nicky. She unveils a jar containing Jake's preserved eyeballs, which she cut out herself, as proof of her dedication to Nicky. Dr. Keller phones Vince at the drive-in, urging him to leave as soon as possible. Moments later, Vince is confronted by Nicky's apparition, and a sudden burst of wind overtakes the drive-in. Meanwhile, Leslie, possessed by Nicky, attacks Dr. Keller. Vince manages to rush back to the house, but finds that Ruby has disappeared. Vince and Dr. Keller flee into the swamp, where they witness Ruby walking toward the water hand-in-hand with Nicky. Suddenly, Ruby screams as she is dragged underwater to her death by Nicky's skeleton.
