Alison Courtland, a wealthy New Yorker, hasn't a clue how she ended up on a train bound for Boston. When she phones her husband, Richard, the police listen in and learn from Richard that his wife has threatened him with a gun. On a flight home, fellow passenger Bruce Elcott falls in love with the married but unhappy Alison. Her husband makes Alison begin seeing Dr. Rhinehart, a psychiatrist. But it turns out that Rhinehart is a fake. He is actually Charles Vernay, a photographer hired by Richard Courtland, who is having an affair with another woman, Daphne, and hopes to get rid of Alison for good.