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Zandra, the daughter of the ruler of the lost city of Pallandria, is saved by Tarzan,the Ape Man, when she becomes trapped on a rock shelf while trying to rescue Tarzan's son Boy.

Later, in their tree house, Boy reads Tarzan a letter from Jane, who is in London visiting her sick mother, describing the Nazi peril.  Soon after, the peril encroaches upon the jungle when Nazi planes fly overhead in search of oil and tin to fuel the war effort.  While parachuting into the jungle, one of the soldiers, Lieutenant Schmidt, is injured and radios a German pilot for help.  Dropping down to a lower altitude to search for the injured soldier, the pilot is blinded by a flock of frightened birds and crashes his plane.  Witnessing the crash, Tarzan saves Schmidt from the jaws of a hungry crocodile and takes the unconscious soldier to his tree house.  Upon regaining consciousness, Schmidt pretends to be British.

Meanwhile, in Pallandria, the Germans, led by Colonel Von Reichart, are naïvely welcomed by Zandra and her father and brother Archmet.  The next day, the Germans seize control of the city and enslave the natives to gather raw materials for the war.  When the Colonel makes sexual advances to Zandra, Archmet comes to her aid and is killed by the Nazis.  Schmidt, meanwhile, has reassembled his radio and is calling Berlin for reinforcements when Cheetah, Tarzan's chimp, steals a coil from the radio and interrupts the broadcast.

Soon after, Tarzan hears gunfire in the jungle and finds Zandra being pursued by the Nazis.  Tarzan sweeps down from a tree and carries her to safety, and when the Nazis try to ford a river to follow them, all but the Sergeant are eaten by carnivorous fish.  Later, when Cheetah steals Schmidt's radio coil again, he shoots at her and chases her into the jungle.  Coming to her rescue, Cheetah's elephant friend pushes the German over a cliff.

As the Sergeant returns to Pallandria with news that Tarzan is in possession of the radio, Zandra tries to convince the Ape Man that the Nazis are his enemies.  In response, Tarzan claims to be an isolationist and refuses to join the fight.  When Zandra insists on returning to her village, Tarzan follows her into the jungle, and in his absence, the Nazis raid the tree house and seize the radio.  As the Nazis begin to torture Boy, demanding that he tell them the location of the coil, Tarzan swings from a tree to his rescue and is shot down by a Nazi bullet.

After the Nazis leave with Boy as their prisoner, Cheetah takes Zandra to the injured Tarzan, and she nurses him back to health.  When Tarzan learns that the Germans have kidnapped Boy, he declares war and hurries to Pallandria.  There, he is captured by the Nazis and sentenced to die before a firing squad the following morning.  Von Reichart also sentences Zandra to death after she rejects his sexual advances.  When the people of Pallandria rally to Zandra's defense, Von Reichart orders that ten percent of the population join Tarzan in front of the firing squad.  Cheetah, meanwhile, slips into the city, reattaches the coil to the radio, and frees Tarzan.

After killing the Sergeant with a knife throw, Tarzan dismantles the German machine gun on the city's tower and then frees the imprisoned people of Pallandria.  Armed with German rifles, the people overthrow their oppressors, sending the Colonel scurrying into the jungle with the radio.  Tarzan follows and, after the Colonel frantically calls Berlin, the Ape Man lures him into a trap with a hungry lion.  Cheetah then takes control of the microphone, and when Berlin headquarters answers the call, they mistake the chattering chimp for Hitler.