When her archaeologist father disappears on an expedition, Wanda sets out to look for him. What she finds is a secret underground world, where no one believes in life on the surface and where she and her father are taken for spies.

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.

With the cyclotram, an atomic-powered rock-boring vehicle, Dr. Jerimiah Morley leads an expedition into a subterranean world.