Late one night, psychology professor Wilma Tuttle is picked up on Pacific Coast Highway in Malibu by Jack Hunter, a kind truck driver. Hunter presumes from Wilma's behavior that she has been stranded because of the unwelcome advances of a date. When Wilma gets home, she shudders, and recalls the day leading up to the evening's events: Wilma gives her university students a written examination on the conditioned reflexes of humans, asking them to describe an unnamed person by his reflexes. After student Bill Perry mercilessly mimics Wilma, she makes an appointment to see him, but then cancels and leaves him a note to consult the dean. Wilma encounters the flirtatious Bill as she is leaving, however, and after he causes her to miss her bus, he offers her a ride, then insists on dinner. As they drive to Malibu, Bill asks Wilma to analyze him, and she tells him that his brilliance and charm conceal a dangerously erratic and poorly controlled personality. Wilma's theory is soon proved correct when Bill drives to an isolated cliffside spot along the coast, changes into his bathing suit to go diving, and then sexually assaults Wilma, who unintentionally kills him while desperately fending off his attack with a tire iron. The next morning, Wilma awakens feeling ill and is obsessed with covering up Bill's death to protect her career. By coincidence, Bill's guardian, lawyer Warren Ford, calls on her and asks her to talk to Susan Duval, a female student who is infatuated with Bill and claims that she is pregnant by him. Shortly afterward, Wilma collapses from pneumonia and awakens a few days later in the university hospital, where news of Bill's death has only recently been reported. A coroner's jury determines that Bill died from drowning and closes the case, but homicide lieutenant Ted Dorgan remains convinced that Bill was murdered. Fully recovered, Wilma attends Bill's funeral and meets Warren, who had sent her flowers while she was ill. After Susan comes under suspicion, Wilma defends her to the dean, claiming that she was only trying to get Bill's attention by saying she was pregnant. A cynical Ted roughly interrogates Susan, who recalls that Bill's last words were that he was going to meet with a "psychlothymiac cutie," a phrase from one of Wilma's exam questions on personality. Wilma becomes frightened when she realizes that Bill described her in his exam, and when she learns that the janitor threw out the note she had left for Bill about their canceled appointment, she replaces it with another, and shows the note and Bill's exam bluebook to Warren. In an effort to disassociate herself from Bill's description of her as a repressed prude, Wilma loosens her hair and dresses less conservatively, and the truck driver who picked her up fails to recognize her. However, while visiting Ted's office, Wilma becomes unnerved when forensics expert Dr. Romley reveals that he has determined that Bill died from a blow to the head, and that he has found splinters from Bill's abalone bucket in his lungs, which suggests that Bill did not drown in the ocean. Wilma becomes hysterical and later, both Romley and Ted are convinced that she is involved in Bill's death. Ted then uses Bill's analysis of Wilma to push her psychologically to the edge. While attending a boxing match with Warren, Wilma is shocked by the men's brutality and cries out, "Bill, you're hurting me." Warren suspects the worst, but nevertheless proposes to her and makes immediate plans to take her to his San Francisco home. Ted forestalls their departure with a subpoena, however, and with Romley's assistance, asks Wilma to participate in a demonstration of how Bill was killed. Wilma reveals her guilt when she picks up the murder weapon and delivers the same type of blows to a plaster cast of Bill's head that she had struck on the night of his death. After Wilma's arrest, Warren defends her at her trial, and in his closing statement insists that Wilma's only crime was concealment, as she killed Bill in self-defense. As Ted listens to Warren's convincing statement, he looks at Wilma's lovestruck face and suspects that he has lost the case.