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In her London apartment, wealthy Margot Wendice discusses with her American ex-lover, Mark Halliday, why she changed her mind about leaving her husband Tony. Several months ago, she explains, Tony suddenly became more affectionate and now, convinced that he cares for her, she plans to remain loyal, despite her love for Mark. Wanting a fresh start in her marriage, she is concerned about an anonymous blackmailer who stole a letter Mark wrote to her, but never picked up the money she paid or returned the letter. When Tony, a former tennis professional-turned-salesman, comes home, he announces that unexpected business matters have pre-empted their evening plans, but encourages Mark and Margot to go out without him. In consolation, he invites Mark to his club's banquet that will be held the following evening. After they leave, Tony calls Capt. Lesgate, feigning interest in a car the man is selling. After manipulating him into meeting at the apartment to negotiate, Tony surprises Lesgate by revealing that they are former Oxford schoolmates and that he is aware of several illegal activities in which the small-time crook has been involved. Intimating that he married for money, Tony tells Lesgate that he stole Margot's letter and blackmailed her to confirm her affair with Mark. Anticipating that she will leave him, Tony wants Margot killed before she takes off with her wealth, and blackmails Lesgate into agreeing to do the killing. As further enticement, Tony promises to pay Lesgate with money that he has been surreptitiously amassing through small bank withdrawals over the past year. As his reputation is spotless, Tony warns Lesgate that attempting to report his proposition to the police will backfire. After Lesgate agrees to cooperate, Tony unveils his elaborate plan: On the following evening, while Tony and Mark attend the banquet, Lesgate is to watch the apartment and when Margot retires, enter using Margot's own house key, which Tony plans to sneak out of her handbag and put under the stairway carpet outside their door. At a specified time, Tony will call, and when Margot gets up to answer, Lesgate, who will be hiding behind drapes near the telephone, is to strangle her. When the deed is done, Lesgate is to whistle into the phone and hang up. Before leaving, he is to leave the garden window open and replace the key under the stairway carpet. The next evening, while chatting over cocktails, Tony's interest is piqued when Mark, who is a television mystery writer, claims that, although he can write the "perfect murder," in real life he would overlook some detail and be caught. Before he and Mark leave for the club, Tony gets the key from Margot's handbag and manipulates her into staying home to clip articles for his scrapbook. At the club, Tony excuses himself from the table, saying he must call his boss, but instead calls home. Meanwhile, after Lesgate has unlocked the apartment door and returned the key to its hiding place, he waits inside the dark apartment. When Margot gets up to answer the phone, Lesgate tries to strangle her with a stocking, but she struggles and stabs him in the back with a pair of scissors that she used to clip Tony's articles. Lesgate falls on the scissors and dies. Tony, on the other end of the phone line, hearing his plans go awry, quickly contrives a different way to accomplish his goal. He talks into the phone and the shaken Margot, recognizing his voice, tells him what happened. After telling her that he will call the police, Tony immediately goes home. Later, at the apartment, while pretending to "protect" Margot from police questioning, he sneaks the key in Lesgate's pocket into Margot's handbag and leaves the love letter where police find it. Other evidence indicates that Lesgate did not come in through the garden window, so Inspector Hubbard, who is in charge of the case, concludes that Margot let in and killed Lesgate, who was blackmailing her. Although Hubbard finds it odd that no key is found on Lesgate's body, Margot is arrested, tried and sentenced to death. Tony, while waiting to inherit her fortune, begins paying his bills with the cash meant for Lesgate. On the day before Margot's scheduled execution, Mark shows up at the apartment urging Tony to invent a story to save Margot's life. As an example, Mark suggests that Tony "confess" that he hired Lesgate to kill Margot, but that she killed her attacker in self-defense. Tony would be safe from prosecution, Mark says, as he could not be convicted for an uncommitted crime. Tony refuses, saying the police would never believe such a story. When Hubbard unexpectedly shows up, Mark hides in the bedroom. While he overhears the inspector inquire about the large amounts of cast Tony appears to have on hand, Mark finds the briefcase of money that was meant to pay off Lesgate. With briefcase in hand, Mark confronts Tony in front of Hubbard, but Tony dismisses Mark's and Hubbard's questions by claiming that the briefcase contains Margot's payoff to Lesgate, which he concealed to hide her guilt. Before leaving, Hubbard reminds Tony to collect Margot's belongings at the police station and then secretly exchanges his own raincoat for Tony's. Later, when the apartment is vacant, Hubbard enters with Tony's key, followed by Mark. Plainclothesmen bring Margot to the apartment and show her in when she is unable to enter using the key in her handbag. As they wait, Hubbard intimates that he needs proof of his suspicions about Tony and has his men return Margot's handbag to the station. Later, Tony arrives, but has no key to get in. Now aware that he has the wrong raincoat, he proceeds to the police station and later returns with Margot's effects, including her handbag. When he again fails to open the door, this time using Margot's key, Tony realizes that the key he took from Lesgate's pocket on the night of the attempted murder was the dead man's own, so he checks under the stairway carpet, retrieves the key he took from Margot's handbag and unlocks the door. Upon finding Hubbard, Mark and Margot waiting inside, Tony acknowledges that his scheme failed and congratulates Hubbard for correctly solving the case.