At the end of an exhausting cattle drive, Gene Autry (Gene Autry) and his faithful sidekick, Frog Millhouse (Smiley Burnette), leave their herd at a train depot, where they meet the lovely Mary Baxter (Frances Grant) and her engineer father, George (Sam Flint). Impressed by their toughness, the Baxters recruit Autry and Millhouse to patrol the unsecured construction site of an important dam-building project plagued by accidents of a deadly and mysterious nature