Mike, the daughter of a railroad section boss, lives with her father and three other children in a remodeled boxcar attached to a railroad work train. Harlan, a section hand, saves the life of one of the children and invites Mike to a village dance, first telling her that he once was a telegrapher but was discharged for allowing the Transcontinental Limited to go through an open switch. Slinky and his gang plot to hold up the mail train, and Mike learns of their plans. They lock her in a boxcar and set it in motion. Harlan rescues her and then wires the nearest government station for help. With the aid of precision bombing by Marine aviators, Mike and Harlan bring Slinky and his gang to justice. Harlan is reinstated in his former job and marries Mike.