Hopalong Cassidy and his pals, Lucky and Pappy, head to the T. D. Rancho to help his former sweetheart Nora get rid of rustlers who are trying to take over her ranch. Nora co-owns the ranch with Tom Dillon, whose daughter Mary is a tomboy. After Mary mistakes Hoppy, Lucky and Pappy for rustlers, they are confronted by Jeff Caffrey, who works for Clay Allison, secret head of the rustlers. Jeff returns with the sheriff and Clay, who accuse the men of cutting the fences and stealing cattle, crimes which Clay actually committed. The sheriff arrests them, and in a court trial Hoppy claims that the accusations are true so that he can infiltrate the gang. He is given probation provided that he and Lucky work for Clay, while old Pappy is sent to work for Ma Caffrey, who tries to shape him up. Hoppy breaks away from the herd one day, leaving Lucky to cover his trail. Jeff catches Lucky and knocks him out, but Mary prevents him from killing Lucky and tries to convince him of Clay's treachery. Clay finds Tom mending fences, and after Tom refuses to sell the ranch to him, Clay shoots him in the back. Jeff witnesses the murder but promises Clay he will remain quiet. Clay plans to claim as his own any unbranded steer on his ranch, including those belonging to Nora, and hires some Texas gunfighters to do the job and also run Hoppy out. Hoppy singlehandedly confronts Clay and the gunfighters, and arranges for Jeff's arrest on suspicion of Tom's murder. In jail, Jeff finds a secret passage out, but when he comes up in the corral he is shot by Clay. Just before he dies, Jeff gives Ma a bullet from Clay's gun, the only gun in the area using those bullets, and the same kind that killed Tom. The gunfighters ride to the ranch to kill Hoppy but are confronted by his friends, who chase them back to town. Pappy leads Hoppy through the secret passage out of the jail. He arrives in time to kill Clay, who was about to kill Ma Caffrey because she knew of his guilt. The town is restored to normalcy and the three cowboys ride off for their next adventure.