Tony Wendice (Ray Milland) is a former tennis pro, living in London with his young wife Margot (Grace Kelly). Wendice doesn't earn very much in his current line selling sports equipment, but his wife is from a wealthy family and can keep him in the style to which he has become accustomed. But when he discovers that she has been having an affair with an American crime writer, Mark Halliday (Robert Cummings), he fears she will leave him and take her money with her. So Wendice contacts a shady old friend from Oxford, Charles Swann (Anthony Dawson), on the pretext of buying a car he has for sale. Swann has a dubious background, a list of creditors and petty crimes, a previous spell in jail and a court martial from the army. Wendice uses the carrot and stick approach, offering Swann £1000 if he carries out Margot's murder, and exposure of his crimes if he doesn't.