The "strangers on a train" are Guy, a successful athlete in an unhappy marriage, and Bruno, an amoral playboy. Bruno suggests a perfect crime whereby Guy can get rid of his wife without being suspected, and he convinces himself that Guy has agreed to it.
Bruno is obviously evil, but what about Guy? Is he is respectable as he claims? Is his wife that bad, or is Guy really a social climber who wants to be rid of his unfashionable spouse in order to acquire a trophy wife? Does he send mix signals to Bruno because he really wants Bruno's plot to succeed? The notion of hidden, shadowy evil is what gives this thriller its power.