In their African jungle home, wild man Tarzan and his mate Jane find their pet chimpanzee Cheetah playing with an old cigarette case, which is inscribed to "Gloria" and dated 1928. While Jane expresses her curiosity about the case's owner, Cheetah and her male chimp friend return to the secluded place where the cigarette case was found, a plane crash site littered with skeletal remains. After romping through the debris, Cheetah returns home with a journal written by famed British aviatrix Gloria James, whose plane disappeared twenty years earlier during a round-the-world flight. Jane tells Tarzan that the journal must be sent to England as soon as possible, but Tarzan flatly refuses to take the book to the trading post at Nyaga Ratu. Tarzan's mysterious refusal annoys Jane, and after she falls asleep, Tarzan goes to the river port and shows the journal to trader Trask and his partner, pilot Dodd. Trask and Dodd inform Tarzan that an Englishman named Douglas Jessup is offering a reward for Gloria's return, as she is the only person who can clear him of a murder charge for which he has been imprisoned for twenty years. Disturbed by this news, Tarzan travels alone to a village hidden in a treacherous mountain valley. There Tarzan, who is the only outsider who knows of the village's existence, tells the tribal leader, the High One, about Jessup and asks if Gloria, who lives in the village, can return to England to save him. The chief grants Tarzan's request, and after he promises not to reveal anything about the village to the outside world, Tarzan leaves with Gloria. The High One's decision angers some villagers, who are convinced that Gloria's departure will mean the end of their isolation. When Tarzan arrives home with Gloria, Jane is thrilled, but is also perplexed by Gloria's youthful appearance. Tarzan refuses to discuss Gloria with Jane, but upon seeing the aviatrix, Trask and Dodd are reminded of a local legend about a hidden city with a fountain of youth. Although Trask claims not to believe in the legend, he hires the brutish Vredak to find the village, sure that such a place would mean great fortune for him. Vredak leads a group of men to the mountains, but as he approaches the valley, he is hit by a flaming arrow fired by a tribal guard. After the other men flee, Pasco, the leader of the dissidents, declares that Tarzan must be killed. Later, Tarzan and Jane receive a letter from Gloria informing them that she has married Jessup and wants to return to Africa with him. News of Gloria's return reaches Dodd, who makes a deal with Trask to follow Tarzan and Gloria to the hidden city. After Tarzan is nearly killed by a valley tribesman, Gloria and Jessup arrive in the jungle in Dodd's plane. Jane is shocked to see that Gloria has aged twenty years and, deducing her secret, tries to convince Tarzan to lead her and the middle-aged Jessup back to the mountains. When Tarzan refuses to lead them, Jane offers to be their guide. Feigning engine trouble, Dodd and Trask then ask to travel with the group for part of the way. Unknown to Jane, Tarzan is following the group and is forced to rescue them when they become trapped in a ravine during a flash flood. After Tarzan announces that the group is turning back in the morning, he sneaks away with Gloria and Jessup. A determined Trask, however, forces Jane to lead him and Dodd to the valley by threatening to kill Cheetah. While Jane guides Trask and Dodd, Tarzan presents Gloria and Jessup to the High One, who decrees that the newlyweds may stay and offers them a drink from the village's fountain of youth. Before Tarzan can leave, however, the dissidents capture him, intending to burn out his eyes. Tarzan escapes from his chains and flees the village just as Jane, Dodd and Trask are being attacked at the valley's entrance. After Dodd and Trask are killed, Cheetah starts a landslide that enables Tarzan and Jane to escape from the fiery arrows. Back at home, the impish Cheetah drinks from a vial of stolen valley water and is turned instantly into a baby chimp.