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One night, Beverly Hills ambulance driver Frank Jessup and his partner Bill are called to the cliffside estate of Charles and Catherine Tremayne. By the time they arrive, Catherine has already been treated for gas inhalation, which the police believe occurred accidentally, but which the wealthy Catherine suspects was deliberate. As he is leaving the house, Frank notices Catherine's beautiful English stepdaughter Diane playing a melancholy piano piece and assures her that her stepmother will be fine. When Diane becomes hysterical, Frank slaps her face to calm her. Confused, she slaps him back, then apologizes. Later, after getting off work, Frank goes to a nearby diner, unaware that Diane is following him in her sports car. In the diner, Frank tries to call his girl friend, Mary Wilton, a hospital receptionist, but gets no answer. Diane then comes in and strikes up a flirtatious conversation with him. When Mary finally calls him, Frank turns down her dinner invitation, claiming that he is too tired. Frank takes Diane out, and over dinner, she tells him that her father is a well-respected novelist but has not finished a book since her mother's death during the war. Diane then asks Frank, a former race car driver who dreams of owning his own garage, about Mary, and he reveals that Mary has been saving her money to help him. The next day, Diane invites Mary to lunch and, while pretending that she wants to contribute to Frank's garage fund, lets her know that he spent the evening with her. Seeing through Diane's tactics, Mary rejects her offer but admits that her faith in Frank is shaken. That night, Mary is about to go out with Frank when he lies again about his date with Diane. Disgusted, Mary rejects Frank and goes out with Bill, a longtime admirer. Later, at the diner, Diane finds Frank, who chastises her for speaking to Mary. When Diane suggests that he drive her car in an upcoming race, however, Frank forgives her and agrees to talk further about her idea. Diane then convinces her parents to hire a chauffeur and, while kissing him on a moonlit drive, persuades Frank to accept the job. Soon after, Diane informs Frank that she has talked to her stepmother about investing in his garage, and he presents Catherine with a written proposal. Although Catherine is suspicious of Diane's motives, she tells Frank that she will consider the offer. Catherine then calls her lawyer, Arthur Vance, for advice, but learns that he is out of town. Later, Diane meets secretly with Frank and tells him that Catherine threw his proposal in the trash. Diane also confides her fear that if Catherine were to find out about their romance, she would fire him and lock her up. Frank tries to reassure Diane that Catherine has no power over her, but Diane insists that Catherine will take her anger out on her beloved, weak father if she is defied. In the middle of the night, Diane then comes to Frank's room and tells him that Catherine tried to kill her by turning on her gas fireplace. Frank refuses to believe Diane's story and orders her back to bed. The next day, Frank stops by Mary's apartment and states that he is leaving his job and Diane. After making a date with Mary for that night, Frank returns to the Tremaynes and starts to pack. Having anticipated his move, Diane cries and begs him to run away with her, showing him her own packed suitcase. Admitting that he loves her, Frank agrees to stay for a few more days so that she can think seriously about the situation. The following day, with Frank gone, Catherine prepares to drive herself to Santa Barbara. As she is about to leave, Charles asks for a lift, and after Catherine puts the car in drive and steps on the gas, the vehicle screeches backward over the cliff. Catherine and Charles are killed in the crash, and following some investigation, both Frank and Diane are arrested for murder. Diane, who stands to inherit all of Catherine's wealth, has suffered a nervous breakdown, however, and is incarcerated in a prison hospital. To help Diane, Vance hires Fred Barrett, a renowned defense lawyer. Just before the trial is to start, Fred convinces Frank and Diane to marry so that he can propose that Diane's suitcase was in Frank's room because they were planning to elope. During the trial, Barrett skillfully deflates expert testimony regarding the car's transmission and steering mechanism, which appears to have been tampered with, and paints Frank and Diane as innocent lovebirds. Frank and Diane are acquitted, but once back at the estate, Frank tells Diane he is divorcing her. Diane finally talks about the jealousy and loneliness she felt when her father married Catherine and the grief she suffered upon seeing their crushed bodies. Despite Diane's remorse, Frank insists he is returning to Mary. After Diane bets Frank her sports car that Mary will not take him back, Frank goes to Mary, who rejects him in favor of Bill. Diane, meanwhile, visits Barrett's office and insists on confessing to the murders, detailing how she asked an unsuspecting Frank to explain the car's transmission. Reminding Diane about the double jeopardy rule, Barrett tears up the confession. Upon returning home, Diane finds Frank packing for Mexico and asks if she can go, too. Frank says no, but agrees to let her drive him to the bus station. After Frank gets in, Diane shifts into reverse, jams her foot on the gas pedal and sends the car over the cliff.