John North, a struggling writer, plans to elope with his married mistress, Susan Wilding, following a quarrel with his wife Carol who is frustrated that her husband refuses employment offered by her father, pointing out they cannot pay bills, and will soon lose their house. She suggests he take the job and write in his spare time until he is established, a thing he refuses even to consider. After meeting Susan in London, he suspects they are being followed, though Susan is dismissive of his concerns. Once they are on the train, he cannot rid himself of his unease. John is guilt-ridden while recollecting the quarrel and feels affection for his wife. Seeing Susan is asleep, he goes out into the corridor, and again thinks he sees the man he believes has been following them. John hears a ticket inspector mention that the train is approaching a point on the line which is close to his house. John thinks he sees Susan's husband further along the corridor, and panics and pulls the emergency
communication cord to stop the train. As the train stops, he passes the still-sleeping Susan and jumps off the train and makes for his house. He tells his wife he has decided to take the job with her father's company and they embrace. Suddenly, they hear the sound of a
train crash nearby. Carol runs to help the victims, while John is stunned as he realises it involves the train he has just left. He walks alongside the wreckage and in a shattered carriage sees a lifeless arm that clearly belongs to his mistress. She and many others in the carriage have been killed in the collision. John says nothing about his presence on the train to his wife, maintaining that he returned from London by bus. In the next day's newspaper, John reads the details of the crash: after he had pulled the cord and the train had stopped, it had been struck by another train, with twenty dead and others injured. Mr Clayton, a
British Railways crash inspector, arrives, and questions John, telling him that they had recovered a document connecting John and Susan. It was found on the man who had been following them, a private detective hired by her husband, both of whom had been identified as among the dead in the crash. However, Carol points out that initials used in the notes could also refer to Susan's husband. Eventually, John admits to his wife that he was on the train and had been running away with another woman, but had pulled the cord and jumped off after changing his mind. When she says she will stand by him, he determines to confess to Clayton, but hears on the radio that the crash was caused by a failed signal rather than his pulling the cord. They still go to tell Clayton, who says that he will not do anything more about John's actions. However, the next day Clayton arrives at the house with Police Inspector Waterson. It has been discovered that, before the train crash, Mrs Wilding had been shot through the heart. Waterson says they suspect that John killed her and then jumped off the train. John denies it, but that evening the police recover a revolver from his garden pond. Fearing he could be hanged for a crime he did not commit, John visits the Wildings' house in London, suspecting that Mr Wilding is still alive; however, Wilding's mother tells him that she identified her son's body. John then goes to the hotel in
Plymouth where he had planned to stay with Susan. He finds Wilding there, who tells him that he was on the train, murdered his wife for being unfaithful, and planted his identifying papers on one of the dead. The two men fight and Wilding shoots John in the head. John finds himself back on the train, apparently recovering from a panic attack in the corridor. Instead of pulling the cord, John returns to Susan and expresses his doubts about what they are doing. This time, she pulls the cord and tells him to go back to his wife. He jumps from the train and arrives at his house, and he and his wife embrace. He hears the sound of a train whistle, but it is just the stopped train moving off again. ==Cast==