Up-and-coming architect, Guy Haines, is traveling to Texas to obtain a divorce from Miriam, pregnant with another man’s child. He has nothing but contempt for her and cannot wait to begin a new life with more sophisticated and loving Anne. On the train, he meets slender, disturbing Charles Bruno, who hates his father. With a lot of booze Bruno goads Guy into confessing his hatred for Miriam. Bruno then proposes a double murder plot, where each would kill the other’s problem.  Appalled, Guy leaves, forgetting his book of Plato.

Not ten days later while vacationing with Anne’s family in Mexico, Guy learns from his anxious mother that Miriam has been murdered. Increasingly tormented that the unstable character on the train may have actually done it, Guy finds his life unraveling as Bruno mails evidence that he is Miriam’s killer, threatens to expose Guy as the instigator, and leaves anonymous letters for Anne. Guy’s work suffers. He drinks heavily and slowly sinks into a state where he realizes his only salvation is to kill Bruno’s father according to the precise plans that have repeatedly been sent. He does.

Guy’s career seems to pick up. But Bruno cannot leave him alone. He turns up uninvited at Guy’s wedding and insinuates himself menacingly into his married life. Guy is miserable, but plays along, aware that he has an impulse to defend Bruno as well as himself. He tells many lies and is wracked with guilt. Anne is worried and suspicious. The two men are bound by their secret, which encompasses a kind of animal attraction rooted in the sensation of having taken a life.

Things could continue indefinitely but for Gerard, the persistent but clever detective who worked for Guy’s father. Having known Bruno for years, he already suspects him of his father’s murder; then he finds Guy’s Plato. To say any more would spoil the gripping conclusion.