Plot. Residing in London, former tennis pro turned sporting goods salesman Tony Wendice and socialite Margot Wendice have a loving, happy marriage,... or so it appears on the surface. In reality, Margot fell out of love with Tony as a travelling tennis wife, and began an affair with American writer Mark Halliday who she still loves. Margot decided to make a go of the marriage when Tony gave up tennis for a more steady job, but she still kept one keepsake from Mark, a love letter, which she carried with her everywhere in whatever handbag she was using, that is until the letter was stolen from her handbag. An unknown person is blackmailing her with the letter, threatening to expose the affair to Tony, something she has told no one except Mark. The blackmailer, which in reality is not blackmail but part of a larger plot, is Tony himself. When first discovering the affair, Tony had thought about killing Mark. But now, his carefully thought out plan is to have Margot killed so that he can live off her wealth. That plan, which only recently coalesced into a beginning to end detailed plot, involves blackmailing old college associate Charles Swann, who Tony has recently learned has turned to a life of fraud and murder. While Tony is out for the evening with a visiting Mark, Swann will kill Margot at the Wendice apartment. Tony will have an alibi and Swann should be able to get away scot-free as he has no formal ties to the Wendices and thus no motive to kill Margot. Although Tony considers the plan foolproof, it does not come off quite as planned, with Margot surviving the attack. Tony has to decide what to do, if he should just hide his involvement in the murder plot, or if he has a Plan B up his sleeve still to get rid of Margot for good. Who may factor into what happens are Mark, who will do whatever to help Margot, most specifically using his creative writing skills, and the lead police investigator, Chief Inspector Hubbard, who may see the crime scene in a slightly different light than the average Joe