In the mid-1950s, Tony Wendice, a retired English professional tennis player, is married to wealthy socialite Margot, who has been having an affair with their friend Mark Halliday, an American crime-fiction writer. Unbeknownst to them, Tony knows about the affair and plots Margot's murder to inherit her fortune, fearing a divorce would leave him penniless.
Tony is also aware that Charles Swann, an old acquaintance from Cambridge University, is a small-time con man with a criminal record. Tony invites Swann to his Maida Vale flat on a pretext, and tells him about Margot's affair. Tony also confides that six months previously, he stole Margot's handbag, which contained a love letter from Mark, and anonymously blackmailed her. After tricking Swann into leaving his fingerprints on the letter, Tony entraps him, threatening to turn him in as Margot's blackmailer unless he kills Margot. With the added inducement of £1,000 in cash, Swann agrees to the murder. Tony then explains that he and Mark will attend a party while Margot stays home alone. At a specific time when Margot is certain to be in bed, Swann will enter the front door, which is always unlocked, and will enter the locked door of the flat with Margot's latchkey, which Tony will hide on the staircase under a carpet. Tony will then telephone the flat from the party and Swann will kill Margot when she answers the call. Swann will whistle over the phone to signal the job is done, then create signs of a burglary gone wrong, and return the key back under the staircase carpet as he is leaving the building.
The following night, Swann enters the flat and Tony calls as planned. When Margot comes to the phone, Swann tries to strangle her with his scarf, but she fatally stabs him with scissors. Upon hearing Margot plead for help instead of Swann's whistle, Tony advises her not to speak to anyone. He returns home, calls the police, sends Margot to bed, and transfers what he thinks is Margot's key from Swann's pocket into her handbag. He also attempts to frame Margot by planting Mark's letter on Swann and destroying Swann's scarf.
Drive-in advertisement from 1954.The next day Tony persuades Margot to hide the fact that he told her not to call the police. Chief Inspector Hubbard arrives to question the Wendices, though Margot makes several conflicting statements. When Hubbard says the evidence indicates that Swann entered through the front door, Tony claims that Swann must have been responsible for stealing Margot's handbag, and made a copy of her key. As Tony intends, Hubbard does not believe the story and arrests Margot after concluding that she killed Swann for blackmailing her. Margot is found guilty of murder and sentenced to death.
Months later, on the day before Margot's scheduled execution, Mark visits Tony, saying he has devised a story for him to tell the police to save Margot. Mark's "story" is very close to what actually happened: that Tony paid Swann to kill Margot. If Tony confesses to Mark’s story, Tony would go to jail for a while, but Margot would be saved. Hubbard arrives unexpectedly, and Mark hides in the bedroom. Hubbard asks Tony about large sums of cash he has been spending around town, tricks Tony into revealing that his latchkey is in his raincoat, and inquires about Tony's attaché case. Tony claims to have misplaced the case, but Mark, overhearing the conversation, finds it on the bed, full of banknotes. Deducing that the money was Tony's intended payoff to Swann, Mark confronts Tony and explains his theory to Hubbard. Tony "confesses" that the cash was Margot's blackmail payment to Swann, which he had concealed to protect her. Hubbard appears to accept Tony's explanation, and Mark leaves angrily. Hubbard discreetly swaps his own raincoat with Tony's. As soon as Tony leaves, Hubbard uses Tony's key to re-enter the flat, followed by Mark. Hubbard previously discovered that the key in Margot's handbag was Swann's own latchkey and deduced that Swann had put the Wendices' key back in its hiding place after unlocking the door. Now, correctly suspecting Tony of having conspired with Swann, Hubbard has developed an elaborate ruse to trap him.
Plainclothes police officers bring Margot from prison to the flat. She tries unsuccessfully to unlock the door with the key in her handbag, then enters through the garden, proving to Hubbard that she is unaware of the hidden key and is therefore innocent. Hubbard has Margot's handbag returned to the police station, where Tony retrieves it after discovering that he has no key. The key from Margot's bag does not work, so he uses the hidden key to open the door, demonstrating his guilt and exonerating Margot. With his escape routes blocked by Hubbard and another policeman, Tony calmly makes himself a drink and congratulates Hubbard.
