Amateur tennis star Guy Haines (Farley Granger) wants to marry Anne Morton (Ruth Roman), the daughter of a senator, and pursue a political career. First, he must divorce his vulgar and promiscuous wife Miriam (Laura Elliott). On a train, Bruno Anthony (Robert Walker) recognizes Guy and knows about his marital situation from the gossip pages. Bruno introduces himself, then proposes an idea for the perfect homicide: he and Guy should "swap murders". Bruno will murder Miriam, and in exchange Guy will kill Bruno's despised father. Each would be killing a stranger. Having no identifiable motive for the crimes, neither would be suspects. Guy humors Bruno's absurd murder plot by pretending to find it amusing. Bruno interprets this as agreement to the scheme. Bruno then borrows Guy's monogrammed cigarette lighter and slips it into his own pocket.