Tarzan and Terk find an injured gorilla named Gobu in the forest. Little do they know, but he is under orders from his leader to lure Tarzan back to Gobu's family. Once the injured gorilla (who Terk has taken a strong shine to) is better, he and Tarzan embark on the long journey to Gobu's home, only to find the evil Tublat, a bull ape who was expelled from Kerchak's family many years before, was waiting there to trap him. Meanwhile, Jane and the Professor attempt to teach Terk to be more lady-like.

The leopard men attack the treehouse and, rather than retrieving Tarzan (as the gang expects), make off with Jane, and gas the others. Once conscious, Tarzan, Tantor, Terk and the Professor set off for Opar to rescue her, only to find Queen La, staffless and at the mercy of a leopard, begging for help. Despite the Professor's warning, Tarzan intervenes and forges a temporary alliance with La to free his wife from La's rebelling minions. However, he is unaware of the true intentions of La and the leopard men.

Terk gets scared when she sees a monster, which is later revealed to be a velociraptor from Pellucidar. The others initially dismiss Terk, but believe her after the dinosaur attacks the Professor's camp as well as Tarzan and Jane's treehouse. Tarzan attempts to find out how the velociraptor managed to get out of Pellucidar and how it can be sent back.

Johannes Niels and Merkus return to the jungle to mine diamonds. Tarzan agrees to this as long as they stay away from the gorillas. Niels and Merkus are threatened by Tublat, who they capture and plan to make money out of him. Tarzan sets out to free Tublat, despite Terk's protests and the past conflict between them.