[Richard Todd], a struggling writer, plans to elope with his mistress, Christine Norden, after an incidental quarrel that morning with his wife, Valerie Hobson, who is frustrated by his refusal to take a job offered by her father, given their precarious finances. After meeting in London, he grows convinced they are being followed both on the street and at a railway cafe where they have a cup of tea, though she dismisses his worries. Once aboard the train, the uneasy mood deepens as they discuss a life together, and he feels the weight of the quarrel while still feeling affection for his wife. When he sees Susan asleep, he slips into the corridor and again thinks he glimpses the man he believes has been tailing them. A ticket inspector’s mention that the train is approaching a stretch near his home only fuels his panic, and he imagines spotting the pursuer farther down the carriage.
In a rush of fear, he panics and pulls the emergency cord to stop the train, passes the sleeping Susan, and jumps off to head for his house just a few minutes away, where he and his wife embrace and he announces he will take the job with her father’s company. Moments later, the town is shaken by a distant train crash—the very train he left behind. Carol rushes to help the victims, while he is stunned to realize the catastrophe involves the train he just fled. He walks along the wreckage and, in a broken carriage, sees the lifeless arm of the woman he had been pursuing, confirming that Susan Wilding did not survive. He chooses not to tell his wife what he witnessed on the train, insisting instead that he returned home by bus.
The next day, the newspaper carries a full report of the crash, and the crisis deepens when a crash inspector named Clayton arrives to question him, revealing a document found on the man who had been tailing them that links John with Susan. The discovery is ambiguous, as the initials could also refer to Susan’s husband. Eventually, John confesses that he had been on the train and running away with another woman, but insists he did not intend harm and tried to turn back. When his wife offers to stand by him, he hesitates to go to Clayton, only to learn from the radio that the crash was caused by a failed signal rather than his actions. Nevertheless, Clayton and Inspector Waterson arrive with the news that Mrs Wilding had been shot before the crash, and the police suspect John of murder. A revolver is later recovered from the garden pond, heightening the fear that he could be hanged for a crime he did not commit.
Fearing the worst, he travels to the Wildings’ house in London, hoping Mr Wilding may still be alive. [Dora Sevening] tells him that she has identified her son’s body, forcing him to confront a grim truth. He then travels to the Plymouth hotel where Wilding reveals that he was indeed on the train and murdered his wife for infidelity, planting identifying papers on one of the dead. The two men fight, and Wilding shoots John in the head, leaving his fate uncertain. Back on the train, John appears to be recovering from his panic, but instead of pulling the cord, he returns to Susan and voices his doubts about their plan. She, in turn, pulls the cord and urges him to return to his wife. He leans toward the window, jumps from the train, and lands at his house to be with his wife. They embrace as a distant train whistle sounds, only to reveal that what they hear is not a train departing but the very train they had stopped earlier, moving on down the line.
