During World War II, Tarzan and his son, Boy, are living on the Great Escarpment, though Jane has returned to the United Kingdom to tend to her sick mother. Jane's letter to Tarzan and Boy describes the ongoing struggle against Nazi Germany. Searching for raw materials in Sub-Saharan Africa to help Germany's cause, a small force of German paratroopers lands and takes over the lost city of "Palandrya" as an advance base and enslave its people. Lieutenant Schmidt is separated from the other paratroopers, but has a short wave radio enabling him to contact his superiors in Berlin. Schmidt convalesces at Tarzan's camp, telling Tarzan he is English.
Zandra escapes after the Germans kill her brother Achmet. Finding Tarzan, she discloses the Germans' presence, and he concludes Schmidt is a German. Tarzan's chimpanzee, Cheetah, with an elephant's assistance, pushes Schmidt off a cliff and he falls to his death. Zandra tries to convince Tarzan to help her people, but Tarzan ignores her, having previously said, "Jungle people fight to live, civilized people live to fight." Zandra plans to return home, but Tarzan stops her.
Several Germans search Tarzan's camp for the radio. They ultimately kidnap Boy, and they presume Tarzan is dead after shooting him out of a tree. Uninjured and hidden from the Germans by Cheetah's monkey brigade, an angry Tarzan shouts, "Now Tarzan make war!" Tarzan infiltrates the lost city, destroying a machine gun, and killing several Germans. He is temporarily captured, joining Zandra and Boy, but is freed by Cheetah and defeats the German invaders with his knife and an elephant blitzkrieg.
In the final scene, Cheetah speaks into the radio; the Germans in Berlin mistake Cheetah's sounds for the rants of Adolf Hitler.
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The U.S. State Department informed Sol Lesser that a Tarzan film would be an ideal way to spread the message of democracy's battle against fascism to the American public.[4] Lesser's first RKO Tarzan film had made the Ape Man a symbol of American isolationism. The film was the highest grossing of Lesser's Tarzan films.[4]
Unlike in previous Tarzan films, the natives are played by whites in South Sea Island costume rather than the black Africans of the MGM films. This use of non-blacks as natives continued for several other Tarzan films in the 1940s.