Tony Wendice, retired English professional tennis player, is married to wealthy socialite Margot, who has been having an affair with 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. Charles Swann, formerly Tony's
Cambridge University classmate, is a small-time con man with a criminal record. Having tracked Swann's activities for some time, Tony invites Swann to his
Maida Vale flat on a pretext. Tony confides Margot's affair to Swann, revealing that six months previously he stole Margot's handbag, containing a love letter from Mark, and had anonymously blackmailed her. Tricking Swann into leaving fingerprints on the blackmail letter, Tony effectively entraps him, threatening to expose Swann as the blackmailer unless he agrees to kill Margot. With the added inducement of £1,000 in cash, Swann agrees to the murder Tony has plotted. Tony and Mark will attend a party while Margot stays at home alone. At a specific time, when Margot will be in bed, Swann will enter the building and open the flat's door with Margot's latchkey, which Tony will hide on the hall staircase under the carpet. When Tony telephones from the party, Swann will kill Margot as she answers the call. Swann will whistle over the phone to signal the job done, then stage a burglary-gone-wrong, putting the key back under the staircase carpet as he leaves. The following night, Swann enters the flat, and Tony rings as planned. When Swann tries to strangle Margot with his scarf, she fatally stabs him with scissors. Hearing Margot plead for help, 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. To frame Margot, he plants Mark's letter on Swann, burning Swann's scarf. The next day, Tony persuades Margot not to reveal he instructed her not to call the police. When Chief Inspector Hubbard questions the Wendices, Margot makes conflicting statements. When Hubbard surmises that Swann entered through the front door, Tony claims Swann must have stolen Margot's handbag and copied her key. As Tony intends, Hubbard is not convinced, arresting Margot after concluding that she killed her blackmailer. At her trial, Margot is found guilty of murder and
sentenced to death. On the day before Margot's scheduled execution, Mark visits Tony, suggesting Tony tell the police "a story" to save Margot that is close to reality: 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. When Hubbard arrives unexpectedly, Mark hides in the bedroom; Hubbard asks Tony about large sums of cash he has been spending around town. Tricking Tony into revealing his latchkey is in his raincoat, Hubbard inquires about Tony's
attaché case. Tony claims he misplaced the case; overhearing this, Mark finds it on the bed, full of banknotes. Deducing that the money was for Tony's payoff to Swann, Mark confronts Tony and explains his theory to Hubbard. Tony "confesses" that he concealed Margot's cash for blackmail payment to Swann to protect her. Hubbard appears to accept Tony's explanation, and Mark leaves angrily. Discreetly swapping his own raincoat with Tony's, Hubbard uses Tony's key to re-enter the flat, followed by Mark, after Tony leaves. Having previously discovered that the key in Margot's handbag was Swann's own latchkey, Hubbard deduced that Swann had put the Wendices' key back in its hiding place right after unlocking the door, instead of upon leaving. Suspecting Tony, Hubbard set up a trap. Plain clothes police bring Margot from prison to the flat. She tries unsuccessfully to unlock the door with the key in her handbag; her unawareness of the hidden key proves her innocence to Hubbard. Margot's handbag is returned to the police station, where Tony retrieves it after discovering his key missing. When the key from Margot's bag does not work, he retrieves the hidden key to open the door, demonstrating his guilt. With escape routes blocked by Hubbard and other policemen, Tony calmly makes himself a drink and congratulates Hubbard. ==Cast==