Hopalong Cassidy and his sidekick, Lucky Jenkins, are sent on a mission by the express company to investigate why the gold shipments of miner Ed Colby are invariably stolen while the rest of the miners' shipments remain untouched. Suspicion points to Colby himself, who, as an ex-outlaw in partnership with the town big shot, Ward Ackerman, stands to gain the total proceeds of the mine by stealing both his and Ackerman's share. Lucky, who is enchanted by Colby's pretty daughter Jane, believes that the mine owner has gone straight and disputes the accusations. Hoppy's investigation also leads him to believe that Colby is innocent, and consequently, when Ackerman insists on appointing his own foreman, Hoppy announces that Jane has already awarded him that position. As soon as Hoppy takes charge, worthless ore starts coming up from the mine, making Hoppy even more suspicious. To draw Hoppy away from the mine so that the thieves can retrieve the gold they have salted away in a tunnel, they stage a raid on his friend Speedy's mine, and Hoppy rides to the rescue. Following the raiders tracks to an old mine shaft owned by Ackerman, Hoppy concludes that Ackerman must be in on the robberies. When Speedy strikes gold, Hoppy decides to set a trap for Ackerman by preparing a new gold shipment for the outlaws. After the bandits rob the stage, Hoppy and the posse track them back to Ackerman's mine, where they capture the gang, thus exonerating Colby of all guilt.