Back to the Stone Age recounts the strange adventures of Lieutenant von Horst, a member of the original crew that sailed to Pellucidar with Jason Gridley and Tarzan who is left behind in the inner world. Von Horst wanders friendless and alone from one danger to the next among the Stone Age peoples, mighty reptiles, and huge animals that have been extinct on the outer crust for thousands of years. But woven among the tales of savage cave men in the country of the Basti, the hideous Gorbuses in the caverns beneath the Forest of Death, and the terrible Gaz is the story of the love this cultured hero feels for a barbarian slave girl who has spurned and discouraged him, working instead toward her own mysterious goal.