Re-edited feature version of serial The Phantom Empire (1935). Singer Gene Autry discovers a race of advanced humans living beneath the earth.

When the ancient continent of Mu sank beneath the ocean, some of its inhabitant survived in caverns beneath the sea. Cowboy singer Gene Autry stumbles upon the civilization, now buried beneath his own Radio Ranch. The Muranians have developed technology and weaponry such as television and ray guns. Their rich supply of radium draws unscrupulous speculators from the surface. The peaceful civilization of the Muranians is corrupted by the greed from above, and it becomes Autry's task to prevent all-out war, ideally without disrupting his regular radio show.

In this Western, comprised of 12 chapters from a serial, Kit leads a group carrying a large gold shipment across the wild West. When the Mystery Riders attack and steal the gold, Kit is the only survivor. He later joins forces with the cavalry to retrieve it.

After Smoke wins the Blake ranch in a poker game, Blake commits suicide and Smoke deeds the ranch to Blake’s young daughter. But the Sheriff is after the ranch and has Smoke arrested for the murder of Blake and then brings in an impostor to pose as the girl’s relative.