The narrative follows Lieutenant Bradley after he parts ways with Fort Dinosaur to explore the barrier cliffs of Caspak. During his perilous trek, he encounters terrifying prehistoric beasts, hostile humanoid tribes, and a bizarre, winged race known as the Wieroos, who inhabit the City of Human Skulls. Bradley is captured by these winged creatures and subjected to their cruel society, discovering their dark secrets and their obsession with human evolution. Along the way, he crosses paths with Co-Tan, a captive Galu girl, and together they forge a desperate alliance to escape the clutches of the Wieroos.
Their flight takes them through underground tunnels, past rivers of the dead, and into the terrifying heart of a temple where the Wieroos worship a false deity. Bradley must rely on his tactical mind, sheer physical strength, and a single pistol to protect Co-Tan and navigate the brutal evolutionary hierarchy of Caprona. After escaping the winged city, Bradley and Co-Tan make their way across the mainland, where they unexpectedly encounter a German submarine and several surviving members of Bradley's original party. The narrative reaches its climax as the united group overcomes hostile forces, rescues the survivors, and secures a daring escape from the primordial island.
