Mine owner George Perez, transporting a load of uranium, is ambushed and left for dead after a fall over a cliff. Badly hurt, he makes his way to town doctor Frank Harding (Leo Cleary), secretly the boss of the outlaw gang working with a foreign government to steal the uranium,who poisons him and passes his death off as the result of an accident. The mine passes into the hands of rancher/businessman Craig Bennett, who makes a claim for the Uranium loss and the company sends Roy Rogers (Roy Rogers) to investigate. He comes to Coronado posing as a cowhand looking for work and gets a job as a member of a line crew that includes Foy Willing (Foy Willing) and the Riders of the Purple Sage(Darol Rice, Al Sloey and George Bamby)working for Bennett's Coronado Power and Light Company. Roy also runs into an old friend, Sparrow Biffle (Pat Brady), who is a helper to Bennett's secretary Pam Reynolds (Dale Evans.) Just as Roy has uncovered evidence linking Dr.