Prof. 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 Lord John Roxton, a fearless big-game hunter, and Ed Malone, a sharp-witted newsman whose publisher advances $100,000 to finance the mission. The journey from London unfolds with sweeping aerial views of Rio de Janeiro and the spectacular Angel Falls, hinting at the grandeur and danger that awaits. The publisher's adventurous daughter, Jennifer Holmes, and her brother, David Holmes, join the team at the head of the Amazon, eager to witness wonders and perhaps profit from the discoveries. Also aboard are Zoological Society bigwig Prof. Summerlee, helicopter pilot Manuel Gomez, and his loyal sidekick Costa.
On the plateau's first night, chaos erupts as a Brontosaurus wrecks the helicopter, plunging the expedition into a world where science meets danger at every turn. As the group presses on, Malone pursues a primitive jungle girl through cobwebs to confront a gigantic spider, killing it and bringing the captive back to camp. The native girl, in time, develops feelings for David, adding a complicated layer to the camp’s fragile dynamics. Jealousies flare as Roxton clashes with the others, and the tension between Malone and Roxton leads to a raw, unresolved fistfight.
The party uncovers the diary of a previous explorer, Burton White, who vanished on the plateau. The diary hints at Roxton’s own history with the place, and White’s pages hint at diamonds waiting beneath the surface. Roxton’s claims that the plateau might conceal a fortune lure the group into further peril, and his prior visits to the plateau three years earlier sow distrust and curiosity among the travelers. The diary also reveals why José Costa remains with the expedition, drawn by the prospect of wealth rather than escape.
As the peril intensifies, Malone and Jennifer are separated from the others and endure a perilous encounter during a clash between a Brontosaurus and another predator. Cannibals seize the group, but a jungle girl—fierce and resourceful—leads them into an underground passage that carries them away from the plateau’s edge. Along their trek, they encounter Burton White again, now living as a blind hermit; the cannibals’ ritual forbids killing the blind, a chilling detail that colors their escape.
The expedition faces a string of hazards: pursued by cannibals, navigating spider-plant dangers, the infamous Graveyard of the Damned, and a lava-pit guardian dinosaur that menaces the diamonds—Costa loses his life to the creature. Manuel Gomez sacrifices himself to break a rock dam, sealing the beast’s fate and saving others in the process.
A volcanic eruption seals the survivors’ escape, with Challenger clutching the egg of a Tyrannosaurus rex as they descend from the plateau. Their financial security rests on Roxton’s practical pluck, who had stashed diamonds in the pockets of his hunting jacket and shares them with the others, a gesture that cushions the group amid their trials. The egg hatches when it is dropped in a moment of chaos, and Professor Challenger resolves to take the infant T. rex back to London with them, ushering in a future where science fiction becomes science fact.
