Dial M For Murder

Tony Wendice, a retired English tennis player, is married to wealthy socialite Margot, an adulteress who has been having an affair with American crime fiction writer Mark Halliday. Unbeknownst to them, Tony has discovered their affair and is planning to have Margot killed so he can inherit her fortune.

Tony is aware that Charles Swann, an old acquaintance from Cambridge University, has become a small-time criminal. Tony arranges a meeting with Swann on a pretext and tells him of Margot's affair. Six months previously, Tony had stolen 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 blackmails him—Swann can either accept £1,000 to kill Margot or be turned in as Margot's blackmailer. Swann agrees to the murder and Tony explains his plan: Tony will accompany Mark to a party, leaving Margot at home. He will hide Margot's latchkey under the foyer carpet and Swann will use it to sneak into the flat and hide. Tony will telephone from the party and Swann will kill Margot when she answers the call. Swann will leave signs of a burglary gone wrong and replace the key under the foyer carpet when he leaves.

The following night, while Margot is in bed, Swann enters the flat and waits. Tony calls and, when Margot comes to the phone, Swann tries to strangle her with his scarf, but she stabs him with scissors and kills him. When Margot grabs the phone and pleads for help, Tony tells her not to speak to anyone. Tony returns to the flat, calls the police, sends Margot to bed, and moves what he thinks is her latchkey from Swann's pocket into her handbag. Before the police arrive, Tony attempts to frame Margot by planting Mark's letter on Swann and destroying Swann's scarf.

The following day, Tony persuades Margot to hide the fact that he told her not to call the police. Chief Inspector Hubbard arrives and questions the Wendices and 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. 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 had already 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 had 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 the adultress. With his escape routes blocked by Hubbard and another policeman, Tony calmly makes himself a drink and congratulates Hubbard.