A singing cowboy stumbles across an ancient underground civilization beneath his ranch.

Gene tries to stop the radium bomb sent by the Queen to kill the riders on the surface.

The Baxters find Gene in time to save him; Argo orders the death of all surface people; battling, Gene arrives at where the Queen is being executed, but is knocked into the path of death-dealing rays.

The Baxters leap to safety but are captured, and brought to Murania; Gene is unable to help, as the door control is disabled; the Baxters switch the door control, but get hit by radium rays.

The Queen orders Gene to tell her who saved him, but he refuses; after Gene sees the kids, he tells the Queen that it was Argo; the Queen is shocked, but forms an alliance with him and orders death to Argo.