In the quiet town of Santa Marta, California, special railroad agent Johnny Douglas bids farewell to his girl friend Lucille Peters' father Pop, a railroad engineer, as he takes a freight and passenger train on its routine run. As the train emerges from a tunnel, it comes to a halt, and the engine car and the mail car explode. Johnny learns about the explosion and that the man in the mail car and the conductor were shot after the explosion, when the mail bags containing a $100,000 payroll were stolen. The rest of the train breaks away and the passenger cars hurtle backward down the tracks until the brakeman manages to stop it. When division superintendent Olmstead hears the news, he assigns Johnny to the case, and Johnny wires Chief Special Agent Wilcox to assist his investigation. After breaking the bad news to Lucille, who is Olmstead's secretary, Johnny launches the investigation by going to the site of the wreck with the sheriff. The sheriff's bloodhounds follow a scent to an abandoned truck, in which Johnny finds some of the robbers' personal belongings and a novel. Johnny realizes that the robbers planned the heist long in advance when he finds a newspaper clipping which reported that some payrolls would be shipped by mail. The robbers, brothers Edmond and Paul Devereaux, meanwhile, hide in a nearby mine shaft while Paul nurses a sprained ankle. Paul recriminates Ed for shooting the unarmed train workers because they saw their faces, but Ed is too dead-set on proceeding with his plan to buy back his family's ranch land and thereby restore the reputation and legacy of the Devereaux family. As part of the investigation, Johnny brings the Devereaux' belongings to University of California criminologist Jerome Bowen, who studies the items for clues about the killers. Although many false leads are phoned in by the public, Johnny finally gets some real clues from the bookstore at which the novel was purchased, and from a librarian who gave the brothers the news clipping. After a couple of weeks, Bowen formulates a description of the brothers, and theorizes that they have worked both as lumberjacks and ranch hands. As Ed and Paul emerge from the mine shaft, Ed shoots a hunter whom they encounter in the hills. The fugitives make their way back to their grandfather's ranch, where they lie to the proud old man and say that they have been given a $100,000 loan by a San Francisco bank. Johnny, meanwhile, continues to investigate by going to all the lumber camps and ranches in the area, and finally encounters ranch foreman Frank Kent, who identifies the Devereaux brothers from a composite drawing. Johnny rounds up a posse with the local sheriff, but Paul's girl friend, Rose McCreary, overhears their plan at her father's barber shop, and warns Paul. Paul and Ed tell her they are wanted because of a fight in San Francisco, and narrowly escape capture by Johnny and the police. Grandfather Devereaux and Rose are shocked when Johnny tells them the true nature of Ed and Paul's crimes. After a week, the brothers are still at large, and Wilcox threatens to bring in another investigator because Johnny has not resolved the case. As they are talking, Jake Rumpler, Jr., and Tad Miller, sons of the murdered train workers, report that they have found the mailbags in the stump of a tree. Johnny and Wilcox hurry to the tree and replace the money with paper, then stake-out the area. Paul arrives shortly after and takes the bag to the mine shaft, unaware that he is being followed by the police. He meets Ed inside, but when the brothers discover the bag is filled with worthless paper, Ed believes that Paul has double-crossed him, and kills him in a fight. Johnny then arrests Ed and wraps up the case. Years later, Johnny is happily married to Lucille and has several children.