From a lost expedition to a tepui in Venezuela, Paula White brings the journal of her father explorer Maple White to the eccentric Professor Challenger in London. The journal features sketches of dinosaurs which is enough proof for Challenger to publicly announce that dinosaurs still walk the earth. Met with ridicule at an academic meeting at the Zoological Hall, Challenger reluctantly accepts a newspaper's offer to finance a mission to rescue Maple White. Professor Challenger, Paula White, sportsman Sir John Roxton, news reporter Edward Malone (who is a friend of Roxton and wishes to go on the expedition to impress his fiancée), a sceptical professor Summerlee, an Indian servant Zambo, and Challenger's butler Austin leave for the plateau.
At their campsite at the base of the plateau, the explorers are shocked when a large rock falls, sent their way by an Homo Erectus perched on top of an overhead ledge. As the crew look up to see their attacker, Challenger spies overhead a Pteranodon (mistakenly calling it a Pterodactylus) killing and eating a young Toxodon which proves that the statements in Maple White's diary are true. Leaving Zambo and Austin at the camp, they cross a chasm onto the plateau by cutting down a tree and using it as a bridge, but it is knocked over by an Apatosaurus (erred to as Brontosaurus), leaving them trapped.