Near the town of Sun Rock, cowboy Roy Rogers and his men witness a gang of masked outlaws hold up a bus and order the passengers to get out. Gang leader Steve Paxton steals a report from one of the passengers, veterinarian George "Doc" Fredericks, which states that local rancher H. T. McKenzie's herd has contracted hoof-and-mouth disease. After Steve kills Doc, Roy tries to arrest him, but he hides inside a chuck wagon belonging to another outlaw named Saunders. Saunders denies Roy permission to search the wagon, and when the two begin fighting, their struggle jolts the wagon and sends it racing downhill. After Steve escapes, Roy tells McKenzie that he is scheduled to appear at a wild west show in Cheyenne, Wyoming. Later, Steve shows McKenzie the report and offers to give it to McKenzie in exchange for ten-thousand dollars. McKenzie agrees and resolves to sell some of the diseased herd to raise the money. Later, grocery man Sparrow Biffle tells Roy that the sheriff has offered a reward for the gang's capture. When Roy and his men decide to visit Roy's old teacher and Steve's stepmother, Dolly Paxton, at the schoolhouse, they are surprised to see young Ruth Shaw instead. Ruth explains that when the parents of her students learned of Dolly's delinquent stepson, they determined that she was unfit to teach their children and dismissed her. Later, Roy and his men go to Dolly's house, but she refuses to let them in. After she gives Steve a gun and rushes him out through the back door, Dolly surrenders, admitting that she has harbored a fugitive. At the jail on Sunday morning, the sheriff releases Dolly and escorts her to church. On the way, she tells him that she adopted Steve after his own mother was killed and his father was sent to prison. At the church, the service has just begun, when Dolly and the sheriff notice Steve standing in the doorway with a gun. He motions for them to come outside, and when they do, Steve escapes with Dolly. Later, Roy and Sparrow uncover a shallow grave containing a diseased cow coated in quick lime. When they find its sick calf wandering nearby, Saunders tries to kill it, but Roy and his men intervene. At the jail, they post Dolly's bail and then ask her to examine the calf. After Dolly diagnoses the disease, Roy and Ruth begin to suspect that McKenzie has known about it all along. If McKenzie tries to drive the cattle to market, they worry, the highly contagious disease could infect the neighboring herds. That evening, as he prepares for the drive, Steve finds Sparrow hiding nearby and shoots him in the leg. Fortunately, Roy is also hiding nearby and shoots Steve, killing him. After Dolly is reinstated at the school, and McKenzie's herd is destroyed, Roy and Ruth leave for Cheyenne.