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. Now on his own, von Horst quickly becomes lost, links up again with the Waziri by accident, and gets lost again when he foolishly goes out hunting on his own.

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 explorer is left poisoned and paralyzed together with other victims, all of them intended as a living larder to feed the creature's young as its eggs hatch. Von Horst passes the time by getting to know a fellow paralytic, the native warrior Dangar of Sari, a member-tribe of Innes' empire. From him, the outer worlder gradually learns the Pellucidarian language. Von Horst's clothing prevented him from receiving a full dose of venom, and he recovers from his paralysis in time to save Dangar from the next hatchling. Shooting the immature trodon, he makes a long strap from its hide, lassos the parent on its next return, and after allowing it to fly off just past the lip of the crater, shoots it in turn. After securing the free end of the strap to the still paralyzed Dangar, he uses it to climb out of the trap, pulling his companion up after him. In the forest at the foot of the mountain he constructs a treehouse to serve them as a secure base while Dangar recovers.