 Rancher H. T. McKenzie (Roy Barcroft) is informed by veterinarian George Fredericks (Emmett Vogan) that his cattle are carriers of the hoof-and-mouth disease and must be destroyed. McKenzie hires Steve Paxton (Byron Barr) to kill the vet and steal his report. Roy Rogers (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 (Elisabeth Risdon), Steve s stepmother. Roy becomes involved in the chase of Steve by Sheriff Holbrook (Montie Montana). Ruth Shaw (Dale Evans) tells Roy that the schoolboard 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.