U.S. Marshal John Carruthers (John Wayne
) is on the hunt for the Polka Dot Bandit (Yakima Canutt
), who has just made off with a four thousand dollar payroll. After teaming up with the local sheriff (George ""Gabby"" Hayes
), the lawmen track the Bandit to a recent supply train robbery. The supplies were headed for Yucca City, and now that the townspeople are facing starvation, they must consider abandoning their homesteads. Out of the blue, town mogul Malgrove (Edward Peil
) steps forward and offers to buy property from anyone who wants to sell—and while it seems like a generous offer, Carruthers smells a rat.
Undercover agent John Martin (John Wayne) is assigned to protect the men building a road in Rainbow Valley from a rough and ready gang who claims ownership of the territory and whose main interest lies in the region's gold mines. When Martin discovers that the gang is plotting to blow up the road with dynamite, he devises a way to use it to his advantage and finish the road while putting a stop to the gang for good. Also starring George "Gabby" Hayes and Lucille Browne.