The tale begins with Bomba, Gibo, Sobrinini and Neram on their way to the Underground River, following the Witch of Snake Island's instructions. For along its route lies a buried chest with specific information on Bomba's early childhood and how he ended up with Cody Carson Casson deep within the bowels of the Amazon Para jungle.
Following a devastating jaguar attack, Neram, Bomba's old friend from his visit to the temple of Jojasta the Medicine Man of the Moving Mountain, is injured and must return to the moloca of Honduras. Sobrinini too is injured, but fatally so. Before she dies though, she regales Bomba (and Gibo) with more information on the Jungle Boy and then performs an opera that rivalled those before the crown heads of Europe. Her death has remained imbedded among the best writing in the juvenile series genre.
Sobrinini reveals in her last moments of lucidity that Bomba's parents were indeed Caboclo, one with African ancestry and the other with Italian. Barton was a painter known all over America and Europe while Laura was an opera singer and friend of Sobrinini. Japazy stole a painting of Laura by (Andrew) Barton and it is that one which Bomba saved from the drowning of Jaguar Island. Japazy (the half breed) had lusted after Laura and asked her to run away with him. She rebuffed his advances and alerted Andrew who challenged him to a duel in which Japazy was wounded but not fatally so. Soon, he fled for refuge in South America.
Meanwhile, the opera company to which Laura and Sobrinini belonged journeyed to Buenos Aires and then Rio de Janeiro on their current tour. There, they and Andrew befriended the naturalist Cody Carson Casson.
After many adventures, Gibo and Bomba literally stumble into a deep and lengthy subterranean cavern through which the aptly named Underground River flowed. The action continues unabated as oddly enough, jaguars and anacondas find their way down but to attack what, I don't know.
When the two warrior/explorers are caught in a flood and thankfully discover a series of stair-like shelves leading above the tunnel floor alongside which the river runs. Slowly, step by step, Bomba and Gibo climb the stairs as the relentless waters do likewise. Eventually they reach the top step and the water continues to pursue them rising to their knees before finally cresting and beginning a slow retreat. But the action doesn't end there as there is an anaconda to contend with before the two can move on.
The two do find the buried chest and lo, it is not underground but alongside the river after it flowed out of the cavern. It's name was still the Underground River. Left unanswered was how Sobrinini discovered this isolated spot and physically buried the chest. In fact, just why she was in the jungle at all has yet to be satisfactorily explained.
Sadly, Polulu has aged in recent books and though he did help save Bomba from the attack of jaguars, he needed help in return. By book's end, Bomba and Gibo begin the return trip to the moluca of Horuhun where hopefully, Cody Casson can sort through the chest and reveal the final details of Bomba's life.
The Amazonian Cycle comes to an awesome conclusion with Bomba the Jungle Boy and the Lost Explorers.