As a young human orphan being raised by gorillas in an African jungle, Tarzan (Harrison Chad) is worried that a fabled monster known as the Zugor (George Carlin) will someday attempt to capture him. He is disappointed that he can’t run as quickly as the other young apes in his family, and his attempts to prove himself keep resulting in chaos. When an accident leaves his ape mother, Kala (Glenn Close), thinking that Tarzan has died, the other apes feel that Tarzan has reached a fitting end. Tarzan believes it best for everyone involved if he runs away.
Alone in the jungle, Tarzan gets pursued by Sabor the leopard right to a rocky place known as the Dark Mountain, inhabited by two hulking, spoiled gorilla brothers, Uto and Kago (Brad Garrett and Ron Perlman), and their controlling, over-protective Mama Gunda (Estelle Harris). But Uto and Kago fear the Zugor as much as Tarzan does, and when the booming call of the monster echoes through the valley, Tarzan is able to escape from them. He encounters a crotchety old gorilla who at first keeps the boy distant, but Tarzan discovers that this old gorilla is actually the Zugor, who uses hollow trees as megaphones to amplify his voice and pretend to be a monster to scare other jungle creatures away from his territory and food. Tarzan uses this discovery to force Zugor into letting the boy stay with him. Thanks to Tarzan’s cheerfulness and helpfulness, Zugor begins to warm up to him as Tarzan continues to try to figure out what he is.
Tarzan’s two best friends, Terk and Tantor (Brenda Grate and Harrison Fahn), come looking for him, but Tarzan does not want to return home with them. It is only when Kala arrives and encounters trouble with Gunda, Uto, and Kago that Tarzan finally realizes what he is supposed to be: a Tarzan, with his own special tricks that no one else in the jungle can do. Terk and Tantor eventually rescue Tarzan and become best friends once again. Tarzan tells Kala that she was right before, and that he is a part of his family. The film ends when Kala gives Tarzan a hug and tells him how proud she is of him for rescuing her from Uto and Kago, after which Mama Gunda punishes them for destroying Zugor’s tree house and Terk and Tantor are finally reunited with Tarzan.