In a lounge car on board a train, a bored Marie Kembar explains to fellow traveler Dr. Valonius that her fiancé chooses not to leave their compartment. Dr. Valonius, who has the ability to predict certain events, tells her a story about a woman on a train who was advised not to leave the train but did so at great cost: Harold Dunlap gets off a train one night in Clayburg, and is followed by a woman, who begins to fight with him. Dunlap stabs her and dumps her body on the observation platform of the departing train, and is seen by a young boy, Mike Bennett, who thinks the body is simply a bundle. Dunlap hitches a car ride with a local newspaperman, McFee, who is en route to cover an impending river flood. However, Dunlap and McFee are forced to return to Clayburg when they encounter closed roads, and McFee recommends that Dunlap stay at Mrs. Thelma Mitchell's rooming house. There Dunlap meets Mrs. Mitchell's niece Jean Maxwell, and discovers that Mike lives there with his mother. After Mike sneaks out to see the swollen river, his mother asks Dunlap to find him. Fearing that Mike will expose him, Dunlap takes the opportunity to try to kill the boy but is thwarted by the arrival of Mike's mother. The next morning, McFee reports that the town is still isolated due to the flooding and that the body of a woman, stabbed through the heart with a nail file, was found on the train. Jean is anxious would like to leave Clayburg and although she suspects that Dunlap killed the woman on the train, she wants to leave with him. Later, Mike realizes that Dunlap is the killer, but Dunlap grabs him and puts him, bound and gagged, in a closet and tells his mother that he has run off. Jean discovers Mike but is knocked out by Dunlap, who takes Mike to a park to kill him. Mike escapes, however, and is found by his mother. Although the roads are now open, Dunlap elects to stay in Clayburg. Back on the train, Dr. Valonius finishes his story and warns Marie not to get off at the next stop, Clayburg. However, when she sees her fiancé, Harold Dunlap, leave the train, she runs after him. They struggle and Dunlap stabs her with her nail file, then dumps her body on the observation platform of the train as it pulls out.