When naturalist Frank Buck is unable to join an expedition through the Brazilian jungle with his friend, Miguel Rojinsky, he suggests that his protegé, zoologist James M. Dannaldson, take his place. Mike leaves his home called Villa Nina to meet Jim in Belem, a town at the head of the Amazon River. Jim and Mike begin their journey by ox cart, which is pulled by bullocks, a breed imported from India many years ago. When they reach the river, the men continue their journey, first by riverboat, then in dugouts. They see a great variety of bird life, including crested blue herons and egrets, whose plumage is protected by the government. Later, they camp at the jungle's edge, and dine on manioc and different meats. For Jim, the main purpose of trip is to capture animals for a zoo. After Jim and Mike secure a young boa constrictor, they see a band of tapirs, an animal that has changed little during the past twelve million years, and iguanas, a lizard that can grow as long as six feet. Over the coming days, Jim catches an iguana, a coatimundi, a mammal related to raccoons, a harpy eagle, an anaconda, a giant anteater and a wild pig, called a peccary. They make pets of monkeys and parrots. Jim and Mike witness a fight between an alligator and a water buffalo, and the struggle attracts a female jaguar. After the buffalo chases her away, Jim and Mike capture her and her babies. While they wait for the arrival of Mike's assistant Clyde, who is bringing permits that will allow them to travel to the area inhabited by the jacaré, or giant caiman, Jim is attacked by an anaconda, but is rescued by Mike. After Clyde arrives with the permits, the expedition continues up the Amazon. Jim captures a South American armadillo and its young, a capybara, the world's largest rodent, and some crab-eating raccoons. The expedition travels through flat, swampy terrain and finally encounters many jacaré. After a fierce struggle, they capture one of the animals. Jacaré, which can weigh up to half-a-ton, will not fight if they cannot see, so after covering its eyes, the men are able to tie it up. Before Jim leaves for home, he captures an ocelot and a water buffalo. Jim gives Mike the ring which Frank earlier had given to him, and bids his new friend goodbye.