A traveler at an oasis hears a man recount an extraordinary youth in which he and a partner tested a powerful mechanical prospector that breached the Earth’s crust and delivered them into a vast inner world. There they encounter prehistoric fauna and an intelligent, rhamphorhynchus-like ruling species that enslaves human tribes. The narrator relates capture and escape, the forging of alliances among rebel humans, the rescue and involvement of a prominent woman called Dian, assaults on the rulers’ temples, and a long struggle of pursuit and survival that culminates in an effort to return to the surface.