In the African jungle, Bomba, a young white man reared in the wild, comes across the camp of zoologist Paul Gordon and releases all the animals from their bamboo cages. The natives tell Paul that Bomba freed the animals, but Paul dismisses the legendary jungle boy as a myth. Later, at his home in the jungle, Paul is annoyed to hear his young son David talk about his friendship with Bomba, and tells the boy he must bring Bomba home to meet the family or they can no longer play together. Only Nona, the servant, believes David, and she follows him when he goes into the jungle to meet Bomba. After summoning Bomba with a series of bird calls, David strips down to a loincloth, and the two friends swing on vines together. Meanwhile, Paul and his wife Ruth are visited by Dr. Charles Langley and his guides, Fred Higgins and Barton. Langley says that recent seismograph records point to the existence of a large volcano in the area, and asks Paul to put together a safari to search for this "lost volcano," which he believes contains the ruins of an ancient city. Paul declines, explaining that he and his wife feel it is time to return to civilization and enroll David in school. Back in the jungle, David tells Bomba about his father's ultimatum, and when Bomba refuses to meet Paul, whom he dislikes for putting animals in cages, the friends sadly say goodbye. David leaves, and Bomba encounters Nona and lets her take the loincloth and knife that David keeps hidden in a tree at their meeting place. After David returns home, Nona comes in and produces the loincloth and knife, which is made of gold and embedded with emeralds. David explains that Bomba found the knife in the hills and has taken him to that secret place but made him promise not to show anybody where it is. Langley again urges Paul to look for the volcano, and when Barton and Higgins offer to take David to school in the city, Paul agrees. David and Nona leave with the men that night, but Barton and Higgins kidnap them, planning to force the boy to lead them to the volcano. Bomba is in a tree when he hears the jeep approaching and sees Nona jump from the moving vehicle and run. Bomba sends Nona home to get help, and Nona, the Gordons and Langley immediately set out in search of David. Later, while the kidnappers are asleep, Bomba unties David and they disappear into the jungle. In the morning, Barton and Higgins capture Bomba and tie him up, but a python drops from a tree and attacks Higgins, and Bomba frees himself and escapes with David before Barton arrives with the jeep. David insists on going for a swim, but they are set upon by a crocodile, and while Bomba is wrestling with the creature, the kidnappers arrive and seize David. When the search party approaches them, Barton opens fire, killing Langley and wounding Paul. Barton and Higgins then escape with David, who reluctantly guides them on foot toward the lost volcano. As the larger volcano near the mountains begins to erupt, Bomba catches up with David and his captors on a rocky ridge and leads them through the lost volcano, which is full of treasure. Barton slips away and greedily begins filling his pockets with jewels, but is soon killed in a rockslide. The search party arrives, and Higgins grabs for Paul's gun, but he falls off the ridge into the path of the molten lava. With David and his family reunited, Bomba bids farewell to his new friends, promising to visit them soon.