One day, Boy, the son of Tarzan and Jane, finds some gold nuggets in the river near their African jungle home.  That night, Jane tells Boy stories about civilization, and he decides to go and look for it.  After leaving his parents a note, Boy rides his little elephant, Buli, and takes the family's pet chimpanzee, Cheeta, with him.

After many adventures, Boy meets a young native named Tumbo and together they escape from a charging rhinoceros.  The children go to Tumbo's village, where they find his mother dying from a fever.  When the tribe sees Boy, they think that he has brought the ravaging fever with him and start to burn him at the stake.  Just then, a truck arrives with some white men carrying guns.  They save Boy, but the natives attack them until Tarzan arrives and stops them.  The men are scientists, led by Prof. Elliott, who are looking for an ancient tribe.

On the way to Tarzan and Jane's tree house, Boy shows the other two scientists, Vandermeer and Medford, a nugget and tells them that there are more in the river.  Tarzan is suspicious of Medford, who is impressed with the "richness" of the area in which he and Jane live.

That evening, Elliott's photographer, O'Doul, demonstrates motion picture projection with a movie that shows an airplane.  An excited Boy then says that Medford promised to buy him an airplane when he gets the gold.  Medford tries to explain things away, but Elliott insists that they will not destroy Tarzan's home by looking for gold.

The next morning, Boy innocently takes Medford and Vandermeer to the river and gives them more nuggets, then tells them that Tarzan knows where there is a mountain of gold.  When Tarzan and Jane arrive, Tarzan angrily orders the men to leave.  Back at camp, Elliott promises to leave as soon as O'Doul, who has come down with the fever, gets well.  Tarzan gives O'Doul some jungle medicine and he improves, but soon Elliott and Boy also become ill.  Medford breaks Elliott's medicine glass as he collapses, then tells the worried Jane that they have better medicine at their encampment.  Although Tarzan does not want to leave, Medford and Jane convince him that he should bring back the medicine.

When Tarzan returns, Medford tells him that Elliott has died and that the now-hidden Jane and Boy will be returned home when Tarzan takes him to the mountain of gold.  Tarzan guides him there and Medford tells him where his family is being held.  As Tarzan starts to leave on a swinging vine, however, Medford shoots at and apparently kills him.

Back at camp, O'Doul pretends to become partners with Medford and Vandermeer and agrees to go with them through dangerous Jaconi tribe lands.  During the journey, Tumbo and Cheeta, who had secretly followed Tarzan, try to get O'Doul's attention.  When the rest of the party stops, they discover O'Doul, seemingly dead and he is able to sneak off with Tumbo to find Tarzan.  Medford and the others are soon captured by the Jaconi, who kill the bearers then take the others down the river in canoes.

Meanwhile, Tarzan, who was merely unconscious, awakens and O'Doul and Tumbo tell them where Jane and Boy are.  Tarzan then races to Jaconi country and, finding their canoes, Tarzan tips them over and fights an alligator who is about to attack Boy.  Unable to get to Jane, Tarzan signals the elephants, who stampede and rout the natives.  When Medford and Vandermer's canoe is tipped over, they are killed by an alligator.

Finally back at their home, Tarzan and Jane secretly fill a gourd with gold and give it to O'Doul, who is returning to civilization.