Architect Guy Haines is on a train to Texas to see his estranged wife Miriam to discuss their divorce. Before long Charles Bruno, a rich n'er do well, sits down opposite him. Haines talks about his problems with Miriam and Bruno talks about his hatred for his father. Before long Bruno makes a suggestion: the two men should "exchange murders". That is, Bruno should kill Miriam and Haines should kill Bruno's dad - and having no demonstrable motive - neither man will be suspected.
Haines strongly opposes this scheme, refuses to participate, and goes on his way. Before long, however, Bruno tracks Miriam down and murders her. He then proceeds to stalk Haines and insert himself into Haine's life at every opportunity - pressuring him to carry out his part of the plan. To say any more would be a spoiler.
The book is a well-crafted psychological thriller with believable well-rounded characters. I wanted to jump into the book and shout at Haines to "get that nutcase out of your life" but of course that would have spoiled the plot. I enjoyed the book. And Alfred Hitchcock made it into an excellent film as well.