In 1898, two adventurers in their mid-20s, American John Cort and Frenchman Max Huber, work in some kind of factory in Libreville, Gabon, in central Africa.

Cort and Huber are left alone with Khamis, a 35-year-old Cameroonian "foreloper" guide, and their adopted native son, Llanga, who is 10 years old.

Now that they have to walk more than 1,000 kilometers to get back to Libreville, the four of them choose to take a shortcut by going through the vast, uncharted Ubangi jungle.

After several weeks of travel and numerous terrifying experiences, the four find the remains of a camp owned by a man named Dr. Johausen.

Additionally, they save a young boy who is more monkey than human from drowning, and finally, they make the most shocking discovery of all: the village of Ngala, which is inhabited by the apelike Waggdi tribe and is situated on a massive platform 100 feet up in the trees.