When Dr. Carter finds that a Sub Earth kingdom is stealing radium from the surface humans, he speaks with Dan Hastings. Along with the doctor's daughter, Gloria Carter, the three go with the lieutenant in his Dan Hasting's Earth boring rocket.
The rocket drills to the underworld kingdom, where they are soon captured by the Sub Earthers and taken before the Sub Earth Ruler. Dan Hastings negotiates, but the ruler is recalcitrant to return the radium. Instead, Dan is thrown in a pit with a monster in it. Hastings uses his raygun to kill the monster before climbing out of the pit. Battling with the ruler's soldiers, the lieutenant inadvertently kills the sub earth ruler in the firefight. Dan then rescues theCarters. The three escape in the rocket, destroying the sub Earth kingdom in their wake.