In 1962, in Allerton, Texas, young Amanda Post awakens in the middle of the night to her mother, Rita's, dying screams, as she is stabbed to death by an unseen assailant. Amanda is confronted by the killer upon finding her mother's corpse in bed, but the murderer lets Amanda go.
Thirteen years later, Amanda receives a phone call notifying her that her grandmother Harriet is on her deathbed. She returns to Allerton for the first time since her mother's murder, arriving at Harriet's large home. There, she is met by Dr. Crawther, as well as Judge Stemple and Claude Kern, the museum owner who hopes to acquire Harriet's historic home after her death. Amanda wishes to have Harriet admitted to a hospital, but Dr. Crawther explains that doing so would go against Harriet's wishes. That afternoon, in private conversation with Claude, Stemple alludes to knowing that Claude murdered Amanda's mother years prior. Later that evening, after Amanda receives several anonymous lewd phone calls, she has her doctor Nick come to examine Harriet. Nick suspects that the medication Crawther administered Harriet has kept her sedated. While Amanda sleeps, she is awoken by the feeling of someone touching her, but Nick dismisses it as a bad dream.
At Claude's invitation, Amanda goes to visit Claude at the local historical society museum, where he shows her a mannequin he has styled to appear as her mother. Angered and disturbed, Amanda storms out of the museum. It soon becomes clear to Amanda that Claude and Stemple are vying for Harriet's home after Stemple offers Amanda a lump sum for it, which she angrily denies. Stemple is persistent, telling her he will return that night after she reconsiders. Meanwhile, Crawther is summoned to meet Stemple at the historical society. Upon arriving, he is bludgeoned to death by Claude, who is cross-dressed as one of the doll-like mannequins on display in the museum.
After, Claude—who, unbeknownst to Amanda, has been responsible for the anonymous calls—phones Harriet's house again. He makes disturbing threats during the call and alludes to her mother's murder. Shortly after, Amanda is met outside by Annie, a local woman who made the original call summoning Amanda back to Allerton. The phone rings again, and Amanda rushes back inside to receive the call; this time, Claude demands that Amanda masturbate while on the line, while he caresses a doll. Amanda soon hangs up, and calls Nick at the hospital for help, but he tells her he is busy and urges her to go to sleep. Shortly after, Amanda unknowingly drinks from a drugged glass of water. As Amanda loses consciousness, Stemple returns to the house, but is bludgeoned by Claude, hiding in the home's foyer.
Amanda awakens some time later, and finds what she presumes to be Nick sleeping in the guest bedroom, only to find it is a mannequin. She also discovers a photo of Crawther's bloodied corpse pinned on the wall in the kitchen. When Amanda attempts to use the phone, she is met by Claude on the line, who continues to make threats and comment on her clothing. Amanda realizes the call is coming from in the house. Moments later, she is confronted by Stemple, who has also regained consciousness; she suspects him responsible until the phone rings again. Stemple goes upstairs to investigate, but is stabbed to death. Nick subsequently arrives, and finds Amanda in a paranoid state. He chases her into the attic, where she pushes him over a bannister to his death. In a daze, she returns downstairs, where the phone rings again. Amanda, driven mad, laughs hysterically.
