In the 1880s, an English couple and their infant son escape a burning ship, ending up on land near uncharted rainforests off the coast of Africa. The couple craft themselves a treehouse from their ship's wreckage, but are subsequently killed by Sabor, a rogue leopardess. Kala, a female gorilla who recently lost her own child to Sabor, hears the cries of the orphaned human infant and finds him in the ruined treehouse. Though she is attacked by Sabor, Kala and the baby manage to escape. Kala takes the baby back to the gorilla troop to raise as her own, an act of which her mate, Kerchak, disapproves. Kala raises the human child, naming him Tarzan. Though he befriends other gorillas in the troop and other animals, including the young female gorilla Terk and the paranoid male elephant Tantor, Tarzan finds himself unable to keep up with them, so he takes great efforts to improve himself. As a young man, Tarzan is able to kill Sabor with his crude spear and protect the troop, gaining Kerchak's reluctant respect.
The film begins with Tarzan and Jane Parker living in the jungle. Harry Holt and his business partner Martin Arlington meet up with them on their way to take ivory from an elephant burial ground. Holt tries to convince Jane, who was with him on his first trip to the jungle, to return with him by bringing her gifts from civilization including clothing and modern gadgets but she tells them she would rather stay with Tarzan.
James Parker (C. Aubrey Smith) and Harry Holt (Neil Hamilton), in Africa on a quest for the legendary elephant burial grounds (and their ivory), are joined by Parker's daughter Jane (Maureen O'Sullivan). Holt, attracted to her, tries somewhat ineffectively to protect her from the jungle's dangers, notably failing to prevent her abduction by the jungle's guardian, the mysterious Tarzan (Johnny Weissmuller) and his ape allies.
The film opens with flashbacks from Tarzan of the Apes to establish the back story. The African expedition led by Professor Porter (Thomas Jefferson) to find Tarzan (Elmo Lincoln), the ape-raised heir of Lord Greystoke, has been crowned with success, and Tarzan and Porter's daughter Jane (Enid Markey) are in love.
Circus workers land an aircraft in the jungles of Africa in search of lions for their show. While trapping lions, the three men meet up with Tarzan (Johnny Weissmuller), Jane (Maureen O'Sullivan) and their adopted son Boy (Johnny Sheffield). Watching Boy's tricks with the elephants, the head of the circus, Buck Rand (Charles Bickford), realizes that Boy would be a great act for the circus. The group is attacked by natives, and it appears that Tarzan and Jane have perished in a jungle fire. The men take Boy on an aircraft back to the United States. Tarzan's loyal chimp Cheeta wakes Tarzan and Jane before they are burned by the fire. Then Cheeta tells Tarzan that Boy has left with the men on the plane.
Jane's two cousins, Eric and Rita, arrive in Africa to tell Jane about a fortune left to her back in their world and to try to convince her to return with them. They are led to Tarzan's escarpment home by Captain Fry (John Buckler), a hunter with an agenda of his own. Jane convinces Tarzan to let her go back with Eric and Rita, promising that their separation will only be temporary. But Captain Fry (unknown to the others) attempts to capture Tarzan to take him back to civilization so he can be put on public display, and actually succeeds in caging Tarzan. Fry's treachery includes making a deal with an unfriendly native tribe to give him food, canoes and protection for the journey back in exchange for his handing over Jane, Eric and Rita for "ju-ju" and taking away the greatest "ju-ju" – Tarzan. Fry's plan goes wrong when the natives capture Tarzan in his cage and all four white people are taken prisoner. Tarzan manages to escape with the help of elephants and Cheeta, and guides what's left of Fry's party through a cave passage filled with treacherous quicksands. Just before they exit the caves to safety, Tarzan forces Fry to go back the way they came as punishment for his betrayal. Fry starts to go back, then seizes a heavy branch to attack Tarzan, but before he can exit the cave he falls into a quicksand bog and is swallowed up. Rita and Eric tell Jane that it is not necessary for her to return with them and that she belongs with Tarzan. The film ends with Tarzan and Jane reunited at their treehouse.
A plane flying to Cape Town, carrying a young couple and their baby, crashes in the jungle. Everyone on the plane dies, except for the baby who is rescued by Cheeta, Tarzan's chimpanzee. Tarzan and Jane adopt the child and name him "Boy". Five years later, a search party comes looking for Boy, because he is the heir to a fortune worth millions. Tarzan and Jane claim the child is dead and that Boy is theirs, but Sir Thomas recognizes Boy's eyes. The younger Lancings suggest leaving Boy and taking the inheritance; when Sir Thomas objects, they say they will take him back and, as legal guardians, still control of the inheritance. Sir Thomas says he'll tell Tarzan, but the rest of the party imprison Sir Thomas in a tent and plan to abduct Boy. Tarzan overhears them plotting; he steals their guns and throws them into a deep lake. Jane arrives the next day and learns what has taken place, and admits that Boy is Greystoke. She persuades Tarzan to retrieve the cache of guns, without which the search party can't survive. Tarzan retrieves them but Jane drops the rope so that Tarzan is trapped.
John, Lord Clayton (Paul Geoffrey), the heir to The 6th Earl of Greystoke, and his wife Alice (Cheryl Campbell) set sail and are quickly shipwrecked on the African coast. After John (or 'Jack') builds a home in the trees, Alice gives birth to a son. Soon after, Alice grows delirious and dies. When Lord Clayton finds her recently dead, the tree house is visited by curious gorillas, and John is killed by one of the apes. The female of the group, Kala, carries her dead infant, but upon hearing the cries of the infant human in his crib, she adopts the boy and raises him among the family of Mangani. The little human child thus grows up naked, wild, and free.
An expedition team arrives on Tarzan's escarpment. By chance, the two villainous members Medford (Tom Conway) and Vandermeer (Philip Dorn) find out that there is plenty of gold on the escarpment. They kidnap Jane and Boy in order to make Tarzan show them the location of the gold. Soon the group is captured by natives, whereupon Tarzan comes to their rescue.
The Bantons (father, Abel (John Carradine) and four sons, Coy (Jock Mahoney), Ethan (Ron McDonnell), Johnny (Gary Cockrell) and Martin (Al Mulock)) rob a pay office in a settlement, killing some people. Coy Banton is tracked down to their camp and taken away by a policeman, Wyntors (John Sullivan). Taking him back to town, Wyntors is killed as two of the brothers seek to rescue Coy. Tarzan appears and kills Ethan Banton. The other brother escapes. Tarzan decides to take Coy to Kairobi for the $5000 reward so he can give it to Wyntors' widow. However, no one in the town of Mantu (same town as beginning of Tarzan's Greatest Adventure) wants to help him. The boat he is waiting for to take him and his prisoner to Kairobi is ambushed by the Bantons, who send the passengers off and destroy the boat.
A native village is robbed, supposedly by natives. Before a man dies, he mentions the name "Slade". Black colouring is found on his hand so they know it is white people who did it, disguised as Africans. Tarzan arrives, remembering Slade from a year previously as a man who let three of his men die during a hunt for a rogue elephant and has a grudge against him. A woman (Angie) turns up, Sanchez' latest woman, flying one of his planes. Later she buzzes Tarzan in his canoe, and crashes the plane so now Tarzan is stuck with her as he goes after Slade. Slade has 3 men and a girlfriend with him. He knows where a diamond mine is and has got his supplies for his trip by stealing them. Tensions run high amongst his men. Kruger is an ex-Nazi and a diamond expert. O'Bannion is an Irish rebel, a drunk and a trouble maker who keeps hassling Dino till Dino decides to kill him but falls foul of a wild animal and quicksand.
Tarzan and Boy, on their way to meet Jane, who is returning from nursing work in England to support the war effort, rescue an "Amazon" woman from an attack by a black panther. During the attack, she drops a golden bracelet which Cheeta picks up. The Amazon woman's ankle is twisted in the incident and she cannot walk. Telling Boy to wait for him, Tarzan carries the woman through a mountain pass to the valley where her city of Palmyria is located. Boy disobeys Tarzan, follows them at a distance, and discovers the location of the city, then returns to wait for Tarzan.
Tarzan sauve une jeune Amazone des griffes d'un tigre, sous les yeux de Boy et de Cheeta. Il la ramène dans la cité perdue de son peuple, endroit qu'il est le seul à connaître. Mais Boy l'a suivi et découvre ce site merveilleux. Jane revient d'Angleterre, accompagnée d'un groupe de savants dirigé par Sir Guy Henderson. Un bracelet en or pur, perdu par l'Amazone, tombe entre leurs mains. Si Henderson n'est là que pour observer des civilisations disparues, d'autres membres de l'expédition se montrent plus cupides. Jane est chargée de convaincre Tarzan de leur montrer le chemin de la cité.
Tarzan and Boy are living on the Great Escarpment, though Jane has returned to England. A small force of German paratroopers lands and takes over the lost city of "Palandrya" as an advance base for the conquest of Sub Saharan Africa. Tarzan continually ignores the requests for help from the helpless and enslaved Palandrians, saying, "Jungle people fight to live, civilized people live to fight."