The story reveals the fate of Wilhelm von Horst, the lost member of the previous book's outer world expedition to Pellucidar, which had been led by Jason Gridley and Tarzan to rescue Pellucidarian emperor David Innes from the Korsars.

The action begins by recapping the incident in which Gridley, von Horst, and Tarzan's Waziri warriors, led by Muviro, are caught up in and separated by a horde of saber-toothed tigers’ cooperative hunt.

In the most powerful sequence in the book,[citation needed] von Horst becomes prey himself when a Trodon, or a pterodactyl-like Dragon, carries him off to its nest in the crater of a dead volcano.

The plot of the novel continues to unfold in its pattern of liberty, capture and escape, with the protagonist's goal imperceptibly altering from rejoining his outer world comrades to romance with La-ja.

The feelings of the principals, while plain to the reader, are masked from their objects of affection by culturally-based misunderstanding, as is typical of Burroughs’ novels, postponing the ultimate resolution nearly to the end of the story.

Within the forest are the labyrinthine caves of the Gorbuses, cannibalistic albinos who, in an eerie touch, are intimated to be murderers from the outer world, reincarnated in Pellucidar and consigned to this place as punishment.

Von Horst pursues the kidnappers, incidentally coming to the aid of a tandor (Woolly Mammoth) wounded by sharp stakes of bamboo, which, Androcles-like, he removes.

Boarded on the family of a tribal warrior, von Horst once again commences plotting to escape, aided by dissatisfied locals, whose support he enlists, and the friendship of Thorek, a member of the tribe who had shared his earlier captivity in Basti.

Innes, it turns out, has taken up the pledge of Jason Gridley at the end of the previous book to rescue the missing von Horst—Gridley himself, anti-climatically, is revealed to have let himself be persuaded by other members of the expedition from the outer world to leave Pellucidar with them instead.