In the jungles of central Africa, Lord Oliver Greystoke is murdered by his expedition companion, Rokov, and his identity is assumed by another Englishman, Edwards. Rokov and Edwards then continue their search for wild man Tarzan, Greystoke's cousin, while communicating with cohorts in Randini using a portable, two-way radio. Tarzan, meanwhile, rescues a young American orphan, Joseph Martin, from an abusive tribe and reluctantly agrees to take him to his home. On the way, Tarzan encourages the resourceful Joey to conquer his fear of lions and shows him how to stare one down. Upon arriving at his treehouse, Tarzan introduces Joey to his wife Jane, then hears Edwards and Rokov approaching. After Edwards introduces himself as Lord Greystoke, he reads aloud from a diary that Tarzan's father kept in the early 1920s, which Lord Greystoke had brought with him. When Tarzan was a baby, Edwards explains, he and his parents lived among the obscure Wazuri tribe. Tarzan's father gained the Wazuris' trust and saw the great diamonds with which they decorated their religious shrines. Edwards asks Tarzan for help in finding the Wazuri, claiming that England needs their diamonds to build machinery, but Tarzan refuses. Jane pleads with him not to turn his back on England and suggests that if she went along, the vicious Wazuri might be willing to deal with them. Though still wary, Tarzan agrees and, with Jane, Joey and their pet chimp Cheetah in tow, guides the safari deep into the jungle. While crossing a river on rafts, Rokov, an avid hunter, insists on shooting at a hippopotamus, which then charges and kills one of the native porters. Furious, Tarzan denounces Rokov and declares that he, Tarzan, is now in charge of the expedition. Later, the group leaves the jungle and crosses a drought-stricken plain. As water supplies run low, Tarzan volunteers to scout for water, and after a long, thirsty night, guides the safari to a spring. There, Cheetah steals Greystoke's passport, which Edwards has been carrying, and, fearing exposure, Edwards and Rokov hunt desperately for it. Tarzan then announces that cannibals are stalking them. When the tribesmen attack, Tarzan battles them single-handedly until the Wazuri show up and scare them off. Although the Wazuri witch doctor denounces all white men as evil, the safari is granted a temporary reprieve from death after an English-speaking tribal elder recalls Tarzan's father and the "good book" from which he taught. The whites are brought before the chief, who commands Tarzan and the elder to find the "good book." After Tarzan departs, Jane finds Greystoke's passport and, enraged, tells Rokov that Tarzan will kill him. Rokov quickly radios his accomplices in Randini, giving them coordinates to land their airplane, then mesmerizes the tribe with some magic tricks, distracting them sufficiently to allow him and Edwards to sneak into their diamond-laden temple. While stealing the gems, Rokov is confronted by the medicine men, whom he stabs to death. Before a guilt-stricken Edwards flees with Rokov, he confesses his deeds to Jane, and fearing for their lives, Jane sends Joey and Cheetah to find Tarzan in the jungle. Tarzan and the old man, meanwhile, locate the hut where Tarzan's parents lived and, after collecting their Bible, head back to the village. Nearby, Rokov and a now delirious Edwards encounter some lions in a ravine. Rokov pushes Edwards into the ravine just as Tarzan approaches, and claims that Edwards fell. Tarzan rushes to help, but Rokov shoots at him, causing him to slip down the hillside, where a large rock rolls on top of him, pinning him. After Rokov leaves Tarzan for dead and races off to meet the airplane, Joey spots Tarzan and, using his new technique, bravely shoos away the lions and frees his friend. Tarzan then tracks down Rokov, and during the ensuing fight, Rokov falls over a cliff to his death. Armed with the diamonds, Tarzan, Joey, the old man and Cheetah rush to the village, arriving just as the chief is about to execute Jane. Tarzan returns the diamonds, and Jane is freed. Cheetah, meanwhile, has found Rokov's radio equipment, and by uttering chimp sounds into the microphone, inadvertently directs Rokov's cohorts to fly into a mountain. In the village, the old man reads a passage from the Bible about sharing and convinces the chief to give Tarzan and Jane half of the diamonds.