Jane finds a lost leopard cub and decides to keep it, but Tarzan won't like having a leopard around.

When the Professor's rival, Samuel T. Philander, visits Africa, he must come up with something spectacular to become famous on before Philander. That's when Tarzan tells them about Pellucidar, an underground world where dinosaurs live. However, Philander follows them there and takes pictures of the dinosaurs. The Professor, having no camera with him, attempts to best Philander by taking something else with him.

Count Nikolas Rokoff hears about a treasure in the valley of the leopards and plans to find it. Tarzan doesn't want to help, until Rokoff threatens to hurt Jane if he doesn't.

A violent thunderstorm has Tarzan, Jane and the rest of the gorilla family seeking shelter in nearby caves. Along the way, Tarzan feels unsure that he can protect the family as well as Kerchak did. Upon reaching the cave and thinking they are safe, they realize they are in the same cave where Tublat lives in. Tarzan is no physical match for Tublat and must use his superior cunning and ingenuity to defeat him.