The story opens as Chet Bullard recovers in a Vienna hospital from a nearly fatal spear wound received during the first successful space flight to the Dark Moon. Walter Harkness, his companion, is fighting public skepticism and the sinister plots of Herr Schwartzmann, who covets their revolutionary starship. Driven by greed and malice, Schwartzmann tricks Chet into leading him to the hidden ship in the Arctic, taking Chet, Walt, and Diane hostage for a perilous journey back into deep space.
Navigating the deadly, vaporous space-serpents that haunt the upper atmospheric layers, Chet proves his mastery at the controls, saving their lives from destruction. Upon landing back on the Dark Moon, they are forced into the rugged Valley of Fires, facing lethal fumeroles, vampiric winged creatures, and the constant threat of Schwartzmann's betrayal. Chet wrecks the ship's delicate controls to prevent Schwartzmann from ever returning to Earth with the vessel, stranding everyone in this untamed world.
The narrative deepens as they encounter the ruins of a gigantic amphitheater and a sinister pyramid inhabited by an ancient, disembodied intelligence known as Krargh, which uses hypnotic ape-men to capture human sacrifices. Despite treacherous landscapes, the betrayal of Schwartzmann, and the sacrifices of loyal allies like the scientist Kreiss, Chet must push his endurance to the absolute limit to rescue Diane and Walt from the horrific fate awaiting them within the pyramid's dark depths.
