Guy Haines, a tennis player, meets Bruno Antony, a stranger, on a train. During their conversation, Bruno proposes a chilling idea wherein two complete strangers exchange murders to solve their respective problems without getting caught. He suggests that Guy kill his father and in return, Bruno will murder Guy's troublesome wife, Miriam. Guy dismisses the conversation as a sick joke, but Bruno takes it seriously and goes on to execute his half of the "deal," killing Miriam.

Guy is horrified when he learns of Miriam's death and realizes Bruno expects him to reciprocate. Bruno relentlessly pressures Guy to fulfill his end of the bargain, threatening to implicate him in Miriam’s murder if he refuses. Tension escalates as Guy grapples with the horrifying situation Bruno has thrust upon him, while simultaneously trying to clear his own name and find a way to stop Bruno. The film builds up to a dramatic confrontation, testing the limits of Guy’s morality and resourcefulness as he tries to extricate himself from Bruno's deadly scheme.