Steve Bolton, a government agent, grants Bronc Masters a lease on the Whispering Range so that Bronc can provide the Army with some of the wild mustangs roaming the range. Soon after Steve congratulates Bronc and prepares to leave town, Bronc angrily storms into the ranch house and announces to his sweetheart, Ann Bradford, and Smiley Burnette, one of his ranch hands, that he is going to town to settle some business. Concerned, Ann and Smiley follow him and witness Bronc accusing Steve of double-crossing him. When Bronc produces a trespassing notice, indicating that someone else has claimed the range, Steve promises to investigate. As Smiley and Bronc ride back to the ranch, a gang of claim jumpers starts to round up the mustangs. At that moment, The Durango Kid, the masked rider for justice and Steve's alter-ego, gallops out of the hills and captures two of the renegades. The Durango Kid then learns that the property is allegedly being leased by Spud Henley, a land developer who plans to sell the horses for glue and fertilizer. When Henley's men report the incident to their boss, Henley informs Steve and the sheriff that Whispering Range has been leased to him by the Secretary of the Interior and presents a document as evidence. After suspending Henley's lease pending an investigation, Steve goes to the telegraph office to wire Washington. One of Henley's men follows him there, and is surprised when The Durango Kid appears. Running out the back door, The Durango Kid jumps Henley in the alley and snatches the lease from his pocket. The next day, Henley visits Briggs, the town clerk, and instructs him to forge another lease. Meanwhile, Steve has notified the sheriff that he obtained the lease from The Durango Kid and intends to ride to the county seat to investigate its validity. Overhearing Steve's plans, Henley sends his men to bushwack him. At the county seat, Steve learns that the Secretary of the Interior is unavailable for consultation. As Steve rides back to town, two of Henley's men are waiting in ambush when, from out of the trees, The Durango Kid appears and knocks them off their horses. Following Henley's orders, the gang stampedes the horses from the Whispering Range through town and into the railroad yard, where they load them into boxcars. When the sheriff refuses to intercede on Bronc's behalf, The Durango Kid and Bronc build a blockade across the railroad tracks, forcing the train to stop on the way to market. As Bronc frees the horses, The Durango Kid learns from the engineer that the Secretary of the Interior is in town. Hurrying to the Secretary's private train, The Durango Kid shows Henley's lease to the official. After the Secretary writes that the document is a forgery, The Durango Kid heads for Bronc's ranch, where Henley has just arrived with the sheriff to arrest Bronc for stealing the horses. When Bronc and his ranch hands flee, Henley knocks the sheriff down and Ann helps him into the house. Just then, The Durango Kid appears, guns ablaze, and shows the sheriff the forged deed. After Henley bursts into the house, The Durango Kid overpowers him, and the sheriff arrests the gang. His legitimacy established, Bronc marries Ann, and Steve, mission accomplished, waves farewell and rides off.