Lieutenant Obergatz had fled in terror from the seeking vengeance of Tarzan of the Apes. And with him, by force, he had taken Tarzan's beloved mate, Jane. Now the ape-man was following the faint spoor of their flight, into a region no man had ever penetrated. The trail led across seemingly impassable marshes into Pal-ul-don - a savage land where primitive Waz-don and Ho-don fought fiercely, wielding knives with their long, prehensile tails - and where mighty triceratops still survived from the dim dawn of time...