As he waits in the Denver train station for the Comanche , which is bound for San Francisco, private detective Michael Shayne meets his old girl friend, reporter Kay Bentley. Shayne assures Kay that he is on an unimportant case, but actually he is escorting Helen Carlson, who is to be a surprise witness in a murder trial. After Shayne makes sure that Helen is safely aboard the train, he bumps into Kay again, and she introduces him to her fiancé, Tom Linscott. Tom is a lawyer with Wentworth Enterprises, and his boss, Caleb Wentworth, is running for governor of California. Shayne comments that Tom will most likely be made lieutenant governor if Wentworth wins, then feigns disinterest as Kay and Tom discuss a newspaper article about the impending murder trial. On the train is Callahan, a former thief whom Shayne arrested once, but who has since gone straight. The paper notes that the trial has been postponed until the arrival of a witness who can clear Callahan and expose political corruption surrounding the case. Soon after, Tom confers with Wentworth's private detective, Carl Izzard, who tells him that Shayne is hiding the witness, and that all of their careers will be destroyed if she testifies. Meanwhile, engineer McGowan drives the train dangerously fast so that he will not be late for his last run, and Helen talks a newly boarded passenger into bringing a drink to her cabin. The man is Everett Jason, a dry goods store owner who has left his overbearing wife and secure life in search of adventure in South America. While Tom tells Kay that Shayne is escorting the witness, Helen confesses her story to Everett. Helen reveals that after serving a year in jail on a trumped-up charge, she was out one night with Wentworth's son when he killed a nightclub owner. Wentworth threatened to revoke Helen's parole and send her back to jail if she testified against his son, and so she disappeared to Denver. After Callahan was framed, however, Helen was found by Shayne, who also threatened to tell the parole board of her whereabouts if she did not return to testify. Everett vows to help her, and the pair decide to run away to South America together. Meanwhile, the $10,000 that Everett has brought with him has been discovered by a porter, who exaggerates the amount until the railroad detective, George Trautwein, becomes convinced that an embezzler is aboard. As Trautwein searches for his criminal, Kay tries unsuccessfully to get Shayne to reveal Helen's location. Shayne is also threatened by Izzard, but before Izzard can use the gun he pulls, the train hits a truck and is derailed. When Everett tries to get help for the unconscious Helen, Kay finds her and after reviving her, pumps her for information. Kay promises to help Everett and Helen escape, then breaks her engagement to Tom, who she realizes has been using her to find Helen. Kay engineers Shayne's arrest, then escapes with Helen and Everett. After Trautwein shoots and wounds Izzard, who he thinks is the embezzler, Shayne escapes and follows Kay and the others to the farmhouse they stop at when their hired car breaks down. After Shayne proves that Kay was interested in helping them merely to get their story, Helen and Everett decide to face up to their responsibilites. Everett returns to his home, while Helen testifies and clears Callahan. Soon after the trial, Shayne takes Kay to a diner where he has gotten Helen a waitressing job. Impressed with Shayne's thoughtfulness, Kay agrees to stay in San Francisco and marry him.