Rancher H. T. McKenzie is informed by veterinarian George Fredericks that his cattle are carriers of the hoof-and-mouth disease and must be destroyed. McKenzie hires Steve Paxton to kill the vet and steal his report. Roy Rogers and the Riders of the Purple Sage (Foy Willing, Darol Rice, George Bamby and Al Sloey) on their way to the Cheyenne Rodeo, detour through Sun Rock so that Roy can visit his old schoolteacher Dolly Paxton, Steve's stepmother. Roy becomes involved in the chase of Steve by Sheriff Holbrook. Ruth Shaw tells Roy that the school board has fired Dolly because of Steve, and that she has turned into a crushed and bitter woman still willing to defend her stepson. Roy discovers a McKenzie calf with symptoms of hoof-and-mouth which puts him on the trail of McKenzie.