In London, 1952, the wealthy Margot Wendice has convinced her professional tennis player husband, Tony, to give up playing to spend more time with her. He now sells sports equipment, and, to Margot, their life together is a happy one. However, he has discovered that while he was on a tennis tour in the United States several years ago, Margot had an affair with an American murder mystery writer named Max Halliday.
Because he wants revenge as well as her money, Tony meticulously plans Margot’s murder. She has no idea that Tony knows of her past affair, nor does she have any suspicion that he planning to kill her. Max comes to London to visit the couple, and Tony uses this as the opportunity to fulfill his plan.
Tony blackmails a man who calls himself Captain Lesgate, a former schoolmate and convicted criminal, to carry out the scheme. They plan what they believe is the perfect murder. But when Margot refuses to play the part of the victim, things go much differently than planned.