Wildlife cameraman Gordon Buchanan goes in search of the rare giant otters. He soon discovers their tracks; they are almost as large as he is. The remote camera he sets to trace the otters unexpectedly captures the elusive jaguar, South America's biggest cat. Meanwhile, canopy expert Justine Evans and the climbing team build a platform high in a tree-top. After a long week of waiting, Justine finds and films a family of red howler monkeys. Insect expert Dr George McGavin crawls into a giant fallen log and soon discovers not only crickets, but also their predator (whipspiders – perfectly adapted for hunting in darkness). Climber and biologist Steve Backshall sets off to explore the Kaieteur Falls, a giant waterfall that lays to the West of the country. His aim is to abseil the 240m to the plunge pool and be the first naturalist to explore this habitat. At the bottom, he is pummelled by the spray but finds a perfect habitat for species of frogs and freshwater crabs, which co-exist in a habitat of wild vegetation.

The team pushes further into the jungle wilderness, searching for the unusual and endangered animals that live there. Not all the animals coming into the camp are welcome. Dr George McGavin is stung by a scorpion and there is a poisonous centipede in the women's hammocks. The Guyanese forest is not a place for the squeamish. George finds the world's largest tarantula – the Goliath bird-eating spider – living in a hole near camp. Steve Backshall joins Dr Phil Willink on the first fish survey of the Upper Essequibo River. Phil is one of the team of scientists cataloguing this remarkably unspoilt forest's natural riches. Their first catch is black piranha, the largest of all the piranha species, then giant Banana catfish and (living in its gills and feeding off its blood) the parasitic vampire catfish (or candiru) – notorious for its ability to lodge inside the human body as well. Strangest of all is a sabre-toothed fish (truly a creature of nightmares), but most exciting for Phil is a much smaller fish that has never been found by scientists before. Guyana's rainforest stretches over the border into Venezuela and meets a range of table-top mountains. Steve attempts to be the first to climb a formidable mountain to explore its unknown habitat. On all sides there is a 350 metre vertical wall of rock that has resisted any attempt by climbers. On the rock face, Steve and the climbing team make camp 115 metres above the ground on tiny collapsible nylon ledges. Gordon and George penetrate deeper into the jungle, with the aim of reaching the headwaters of a river rumoured to have a wide abundance of giant animals that show little fear of people. But first, they have to find their way round a ferocious set of rapids.

Climber Steve Backshall reaches the summit of a formidable table-top mountain, up a vertical cliff-face almost as high as New York's Empire State Building. The summit is covered in carnivorous plants, such as pitcher plants and sundews; the soil has been washed away by the incessant rains; and the plants need to catch insects to grow. Steve finds tiny frogs and spots the rare orange-breasted falcon, hunting for the many swifts that live on the cliffs. On the Rewa River, Dr George McGavin and Gordon Buchanan have a battle on their hands to reach the headwaters. The rapids are impassable, so they have to haul three boats and all their kit through the jungle in the sweltering midday heat. Eventually, they reach the headwaters and find a family of giant otters, a Capybara (the world's largest rodent) and, hidden in the undergrowth, a five-metre long anaconda digesting its dinner. Gordon then makes his most spectacular find – a magnificent Harpy eagle is perched in a tree with a dead monkey in its talons. These are the most powerful eagles in the world and they specialise in snatching monkeys out of the canopy. At the end of the journey George meets Bharrat Jagdeo, the President of Guyana, to present a report on the expedition. The President hopes the forests can be left standing to protect wildlife and to help fight climate change.