Prospectors have discovered a natural cement mine that yields material of astonishing durability. But greedy contractor Sam Brady (Robert Livingston) enjoys a monopoly on the transportation of crushed rock, which is being used to build a local dam. U.S. Marshal Gene Autry (Gene Autry), however, discovers that Brady has used this method before, and his dams always burst shortly after construction, flooding entire towns. Despite Brady's power, Autry campaigns to have the new dam made of cement.