Lula May Rivers must get to Cheyenne, Wyoming to get her niece, but a robbery left her broke. Dressed as a boy who calls herself Lou, she stows away in the baggage car of a train headed that way. Unluckily, she's found. U.S. Marshal Gannon Calloway lets Lou stay aboard. He's expecting a robbery and, as an expert with a whip, Lou can help. Then Gannon learns that a killer he put in prison has escaped and is headed to Cheyenne seeking to kill a female witness he blames for his conviction. What will Gannon do when he finds out the boy Lou is a beautiful woman, and the witness the convict wants dead? Can they work together to catch him? And maybe learn along the way that love can come despite one's best intentions?