After the events of Tarzan: The Lost Adventure, our hero is trapped in a cave system deep within the earth's crust. Being unable to return to the surface, Tarzan instead heads deeper, eventually emerging in Pellucidar—where he is promptly knocked on the head and kidnapped by Korsars.

The Korsars, under the command of Brola, have established a base on the island of Thula. They are building a navy with which to overthrow the Empire of Pellucidar. David Innes (emperor, in case you forgot) has gotten wind that something is up, so he and Ja of Anoroc sailed off to investigate and promptly got themselves kidnaped by Brola—but not before sending off a Gridley Wave message for help from the Empire.

Brola throws Tarzan and David Innes into the arena for his amusement. First they fight a pack of hyaenodons, barehanded. Next Brola makes them fight his other gladiators: a Horib, a Sagoth, and prisoners from Sari, Thuria, and Amiocap. It's a six-to-two fight, and their enemies are armed, but of course Tarzan and Innes don't go down without a fight. They are losing when the cavalry arrives: hot-air balloons from Sari drop bombs, marking the first aerial bombardment the Korsars have ever experienced. This technological innovation is, of course, courtesy of Abner Perry. The Korsars are quickly defeated, Brola is killed in battle, and Tarzan, David Innes, and Ja return safely to Sari.

Tarzan decides to stay in Pellucidar. Its savage primeval frontier is more to his liking than the rapidly modernizing world of the outer crust. All he desires now is to bring Jane to Pellucidar. Then his happiness will be complete.