Frank (Mitchum) is a former race car driver now driving an ambulance, who has ambitions of opening his own race car garage shop. He also has a pretty blonde girlfriend, a nurse named Mary (Mona Freeman). His ambulance is called to the Tremayne residence, where Grande Dame Catherine Tremayne (Barbara O'Neil) has been overcome by the fumes from a fireplace. Catherine turns out to be fine—she was rescued from the fumes by her husband Charles (Herbert Marshall) in time, and doesn't even need to go to the hospital. Her gorgeous stepdaughter Diane (Simmons) notices handsome Frank and takes a fancy to him.
Diane follows Frank's ambulance, follows Frank to a diner, and gets him to take her out. She then arranges a meeting with Mary in which she tells Mary about their date, causing Mary to break up with Frank. Diane gets her hooks further into Frank by getting him a job as chauffeur at the Tremayne mansion. It eventually becomes clear that the business with the gas in the fireplace wasn't an accident, and Diane has murderous intentions towards her stepmother.
Produced by Howard Hughes, who had an unhealthy obsession with Jean Simmons. She was cast in this film when Hughes refused to let her do Roman Holiday because she wouldn't date him (Simmons was married). Simmons later retaliated by suing Hughes and RKO to get out of her contract.
- Age-Gap Romance: Diane is a teenager when she starts an affair with Frank, who looks to be in his mid- to late 30s and leaves his more age-appropriate girlfriend in the process.
- Amoral Attorney: The attorney gets Diane off scot-free and later when she says that she really did commit the crime, he tells her to forget about it because of double jeopardy. He doesn't seem bothered at all by the revelation that he got a murderer acquitted.
- Betty and Veronica: There's Betty in the person of Frank's blonde, attractive, and entirely non-criminal girlfriend Mary, and Veronica in the person of dark, sensuous, and murdering Diane.
- Bitch in Sheep's Clothing: Diane repeatedly plays up her image as a gentle, well-mannered ingenue, but both Mary and Frank see right through her and call her out as being a manipulative schemer, treating them like they're idiots.
- Coincidental Broadcast: A radio news report about Diane and Frank's acquittal is perfectly timed for Diane's return to the house after confessing to her lawyer.
- Daddy's Girl: Diane is very close to her father, and protective of his creative interests.
- Downer Ending: Diane drives the car off the cliff, killing herself and Frank.
- Driving a Desk: Painfully obvious in some of the scenes where Frank is driving Diane around.
- Femme Fatale: Diane is a complicated case: she plays this straight in the beginning and ending, using sex to get someone to help her commit murder in order to get an inheritance and killing both herself and her lover when things won't go her way. However, she isn't the heartless schemer that a typical Femme Fatale is, feeling so depressed about inadvertently killing her father that she ends up hospitalized and repeatedly trying to confess and assert Frank's innocence.
- Friendless Background: Diane states that she has no friends, and never had any even as a child.
- Get a Hold of Yourself, Man!: Diane is in (probably faked) hysterics after Catherine's gas incident. Frank slaps her to snap her out of it. She looks him in the eye and then slaps him back.
- Heel Realization: Diane admits to Frank that she understands now that her hatred toward Catherine is because she viewed her as an intruder, but once she saw their bodies, she realized that Catherine did love him and did nothing to earn Diane's dislike of her. This leads to a HeelFace Turn where she tries to do the right thing, but she can't make any progress.
- If I Can't Have You : The ending where Diane reverses the car off the cliff deliberately with her and Frank in it, killing both of them.
- One Head Taller: Mitchum looks like he could put Simmons in his front pocket. This adds an interesting dynamic to scenes where she is manipulating and controlling him.
- Self-Made Orphan: Although Diane only wanted to murder her stepmother and is pretty upset when her father dies in the accident as well.
- Smart People Play Chess: Diane's intelligence as well as her ability to plot is underscored by a scene where she beats her father at chess.
- Spousal Privilege: Frank and Diane get married so they can't be forced to testify against each other in the murder trial.
- Sweater Girl: Diane wears a series of tight tops throughout the film.
- Vehicular Sabotage: The car is tampered with so that it will go into reverse when the gear shift is set to drive. Given that the family lives and parks on the edge of a cliff, it doesn't take much to make this effective.
- Writer's Block: Charles, a writer, can't write anymore. Both he and his daughter blame this on Catherine. Frank cynically posits that Charles, now rich thanks to Catherine, has let himself get complacent and spoiled.
- Wicked Stepmother: Downplayed. Catherine is pompous and can be obnoxious, such as rejecting the garage funding, but she's not even close to being the monster Diane believes she is.
- Yandere: Diane is very jealous and tells Frank that she isn't like Mary, because Mary would always live with the uncertainty of whether Frank really committed murder, while Diane wouldn't care one way or another.