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.

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.

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.