Bob, a train driver with the Great Western Railway, arrives back in London in a particularly irascible mood and is curt with Charlie, his stoker. His wife Emily welcomes him with open arms, but he is much more interested in the letter from his employers that awaits his return. He is devastated when he reads that they have decided to dismiss the appeal to postpone his retirement after forty years of service. She does her best to console him and he apologises for his brusque behaviour. She manages to calm him down and he admits that he thinks his head has been playing tricks on him. Emily goes to the kitchen to write a letter to Charlie.
Pip and Daisy, two confidence tricksters, are on the run from the police. When a constable comes to their flat, they create a diversion and then decide to catch a train and leave town. At the train station they come across Goddard, who appears quite drunk. He buys Daisy a drink in the bar and she distracts him while Pip tries unsuccessfully to pick his pocket.
Tom goes to see his fiancée, Diana, to apologise for an argument they had over Gerald, an admirer of hers. He finds a note telling him that she and Gerald are getting married and then catching a train to go on their honeymoon. Tom drives off to the station in an attempt to stop them.
Bob overhears Emily confiding in Charlie and making him promise not to reveal that he has come to see her. After he leaves, Bob bids her farewell and goes to the station, convinced that she and Charlie are having an affair behind his back. Tom arrives just as the train departs. He gets back in his car, determined to catch up with them at the first train stop.
Diana reveals to Gerald that she is a bit apprehensive about their marriage since they have only known each other for a month, but he reassures her. Goddard continues to drink with Daisy's encouragement. Pip runs into Gerald and recognises him as a fellow con man. Charlie tells Bob that the train is going too fast, but is ignored. When he presses the point, Bob accuses him of having an affair with his wife and of having come to see her that day. Charlie denies it, which infuriates Bob, who pulls a gun on him. He tells Charlie that he intends to crash the train and take everybody with him.
Gerald realises that Pip has stolen his wallet and goes to find him. Goddard, who is actually an undercover policeman, discovers the three confidence tricksters. He is about to arrest them when the speeding train lurches forwards, allowing Gerald to knock him out. Gerald plans to kill Goddard and throw him from the train, but the other two refuse. The train hurtles forwards much too fast and fails to make its first stop. Charlie escapes from the engine but is shot in the shoulder by Bob.
Sir Wilfred Rhodes, a doctor, is called to attend the wounded man and Charlie tells him that he must stop the train before it crashes. He manages to reach the engine and eventually is able to calm down Bob and show him the letter Emily had written to Charlie, expressing only her love and concern for her husband. Bob stops the train just before it crashes into the buffers. Goddard lets Pip and Daisy get away in return for saying his life. Gerald tries to escape, but Tom has chartered a small plane and arrives in time to see that he is arrested for bigamy. Bob recovers from his mental breakdown and settles down to enjoy a long retirement with his forbearing wife.
