Professor Challenger, a celebrated biologist and anthropologist, reports to the London Zoological Society that he has discovered living specimens of supposedly extinct animals, including dinosaurs, on an expedition to the Amazon Basin and up a barely known plateau. He challenges the Society to sponsor a second expedition to the plateau. Much to Challenger's dismay, he attracts a few very unscientific people to join him on his second journey to the Amazon. This expedition group includes big-game hunter Lord John Roxton, and newsman Ed Malone whose publisher advances $100,000 to pay for the expedition. The publisher's adventurous daughter Jennifer and son David join the group at the head of the Amazon. Also in the group is Zoological Society bigwig Professor Summerlee, helicopter pilot Manuel Gomez, and his sidekick Jose Costa. During the first night on the plateau, a dinosaur wrecks the helicopter. As the expedition proceeds, Malone chases a primitive jungle girl through cobwebs to a giant spider, killing it and bringing her back to camp. The native girl later falls for David. Roxton argues with the others, and jealousies over Jennifer lead to a fistfight between Malone and Roxton. They discover the diary of a previous explorer, Burton White, who was lost on the plateau. Roxton is mentioned several times in the diary. Roxton reveals that he had visited the plateau three years before, and claims the plateau holds a bounty of diamonds. This motivates Jose to stay with the party instead of striking out on his own to escape from the plateau. At one point, Malone and Jennifer are separated from the others and have a near-death encounter with two battling dinosaurs. Cannibals capture the party, but before they can become dinner the jungle girl leads them to an underground passage that leads down off the plateau. Along the way, they encounter Burton White, now living as a blind hermit (The cannibals have a taboo against killing the blind). They encounter more obstacles - pursuit by the cannibals, spider plants, the "Graveyard of the Damned," and a dinosaur in a lava pit guarding the diamonds, which kills Costa. Gomez sacrifices himself while breaking a rock dam, killing the dinosaur. During a volcanic eruption, the survivors of the Challenger party escape from the plateau, Challenger carrying the egg of a Tyrannosaurus rex. The financial security of the party is secure because Lord Roxton filled a couple of the bellows pockets of his hunting jacket with diamonds, and shares them with everyone. The dinosaur egg hatches when it is dropped by accident, and Professor Challenger decides to take the infant T. Rex back to London with them.