In his first starring role, Gene Autry must perform daily on Radio Ranch or forfeit his contract. Meanwhile, local kids Frankie (Frankie Darro) and Betsy Baxter (Betsy Ross King) establish a group of Junior Thunder Riders to emulate a mysterious band of horsemen that seems to vanish into thin air. In reality, the real Thunder Riders disappear 25,000 ft. below the earth's surface to the "Scientific City of Murania," an underground empire lorded over by Queen Tika (Dorothy Christy), a blonde Amazonian who constantly compares her superior society with that of the pitiful world above. But Gene's broadcasts draw too many curious onlookers, among them Professor Beetson (J. Frank Glendon) and a group of crooked scientists who will stop at nothing, including murder, to get their hands on Murania's wealth of radium. While Queen Tika is busy preventing an insurrection lead by the evil Lord High Chancellor (Wheeler Oakman), the scientists do their level best to keep Gene from performing his daily broadcast, which includes such favorite Autry tunes as "That Silver-Haired Daddy of Mine" and "I'm Getting a Moon's Eye View of the World". Comic sidekicks Smiley Burnette and William Moore add to the overall fun with their rendition of I'm Oscar, I'm Pete" and other comical selections. The Phantom Empire has been credited with inspiring not only Republic Pictures' similar Undersea Kingdom (1936) but also Universal's superior Flash Gordon and Buck Rogers serials and was remade as part of the short-lived 1979 television series Cliffhangers. No less than two reedited feature versions of The Phantom Empire were released in 1940, Men With Steel Faces, distributed by Times Pictures, and Radio Ranch, distributed by Nat Levine and carrying the now defunct Mascot label.