Tony Wendice (Milland) learns that his wife, Margot (Kelly) had an affair but even though it's over, he decided to kill her but chose to bide his time. He waited until Mark Halliday (Cummings), his wife's boyfriend, returned to town. He then placed his plan in order. He summoned a man called Charles Alexander Swann (Dawson), whose reputation is a bit shady, and reveals that he knows his secrets but also tells him about his wife and her boyfriend and that they are each the beneficiary of their estates. He gets the man to agree to kill his wife and he has laid out what appears to be perfect plan. Only thing is the man was the one who was killed. Wendice trying to cover everything up, decides then to make it appear that Margo had an ulterior motive for killing the man. And it appears to be working except Halliday doesn't believe she's guilty and the police uncover a few anomalies.