When American writer Mark Halliday visits the very married Margot Wendice in London, he unknowingly sets off a chain of blackmail and murder. After sensing Margot's affections for Halliday, her husband, Tony Wendice, fears divorce and disinheritance, and plots her death. Knowing former school chum Captain Lesgate is involved in illegal activities, Tony blackmails him into conspiring to kill Margot. When she kills Lesgate in self-defense, Tony implicates her as being guilty of premeditated murder. Halliday must out-strategize Tony to save Margot's life.