Two months have passed since the conclusion of the previous novel, Tarzan the Untamed, in which Tarzan spent many months wandering about Africa wreaking vengeance upon those who he believed brutally murdered Jane. At the end of that novel Tarzan learns that her death was a ruse, that she had not been killed at all.

In attempting to track Jane, Tarzan has come to a hidden valley called Pal-ul-don filled with dinosaurs, notably the savage Triceratops-like Gryfs, which, unlike their prehistoric counterparts, are omnivorous and stand 20 feet tall at the shoulder. The lost valley is also home to two different adversarial races of tailed human-looking creatures: the hairless and white skinned, city-dwelling Ho-don and the hairy and black-skinned, hill-dwelling Waz-don. Tarzan befriends a Ho-don warrior, and the Waz-don chief, actuating some uncustomary relations. In this new world Tarzan becomes a captive but so impresses his captors with his accomplishments and skills that they name him "Tarzan-Jad-Guru" (Tarzan the Terrible)...