Lorna Drake, British co-owner of the LD ranch, arrives at the ranch with her aunt Martha and fiancé Ronald Harwood, after having been rescued from her runaway horses by Hopalong Cassidy, the foreman. Insisting on total control over everyone, Lorna sees Hoppy as a challenge to her authority, and when she insists he take her for a tour, although he has other work to do, he quits. Lorna has Hoppy arrested when she learns that he has no bill of sale for his LD ranch horse, and then has him released into her custody. Recognizing that she is playing a game, Hoppy pleasantly takes orders from her. In the meantime, however, ex-convict Scar Lewis has returned to town and, intent on revenge against Hoppy for his imprisonment, uses Ronald's jealousy of Hoppy to convince Ronald to kill Hoppy. Ronald fails in his attempt, so Scar and his accomplice Doc Galer, plan to lure Hoppy and Lorna to a cabin down canyon from the dam, then dynamite the dam. To effect this plan, Ronald uses Hoppy's sidekick Baldy to take Lorna to the cabin, leading Baldy to believe it is a romantic rendezvous. Realizing something is up, Hoppy goes to the dam with his friend Lucky Jenkins where they are forced to shoot Doc after Ronald confesses. Before he dies, Doc shoots the wick on the dynamite and ignites it. After Hoppy separates the wick from the dynamite to prevent an explosion, a brush fire starts. Ronald removes all the dynamite while Hoppy rescues Lorna from the burning cabin. Lucky and the LD ranch hands arrive during a gunfight between Hoppy and Scar and his men, which results in Scar's death and his gang's surrender. For the first time, Lorna listens to an order from Hoppy when he sends her back to the ranch. Later, Lorna and her aunt leave for California, and Ronald stays on at the ranch to learn more from Hoppy.