In a train journey, two men meet by chance, and what begins as a friendly conversation soon turns into a dangerous proposal. Guy Haines, a struggling architect, is trapped in an unhappy marriage with a woman who refuses to grant him a divorce. On the train, he meets Bruno Antony, an eccentric and charismatic millionaire who has a macabre idea to free Guy from his bondage. Bruno suggests a "swap" of murders: he will take care of Guy's wife while Guy would dispose of Bruno's abusive father figure. At first, Guy laughs off the proposal as a joke, but Bruno takes it very seriously and is willing to carry out the plan.
Guy's life turns into complete chaos when Bruno fulfills his part of the deal, murdering Guy's wife and forcing him to face the consequences of his complicit behavior. From that moment on, the tension escalates into a spiral of psychological terror, where Guy feels increasingly trapped in a web of manipulation and madness. As he tries to clear his name and distance himself from Bruno, he realizes that escaping this deadly game will not be so easy. The film explores themes of obsession, morality, and the thin line between reason and madness, all under Hitchcock's masterful direction, which keeps the audience on the edge of their seats until the very end.