The boisterous, arrogant professor Challenger, a reputed biologist and anthropologist, dares the London Zoological Society to mount an expedition to verify his spectacular claim, without physical proof, that his previous expedition to the Amazonian basin found live dinosaurs. Apart from him and his 'socialite' counterpart, professor Summerlee, it consists of experienced discoverer Lord Roxbury, the young reporter Ed Malone -who got publicly struck down with Challenger's umbrella at his arrival- and Jennifer Holmes, Malone's news agency's boss's daughter, essentially as conditions for putting up the money. In Brazil they are joined by Jennifer's brother David and local 'guide' Manuel Gomez. They soon discover the dinosaurs are real and dangerous, like giant spiders, but loose their helicopter and thus are desperate for a way down from the isolated plateau. They learn Roxbury knew about the fate of Burton White, an explorer whose diary they find, thus presume to be dead, in search of diamonds, and confirmation of the local tribe being the lethal guardians of the plateau's secrets, but also get surprising help..
