While in the Amazon rainforest, Professor George Challenger shoots an animal he believes to be a pterosaur. Returning to England, Challenger crashes a lecture at the Natural History Museum held by his rival, Professor Leo Summerlee. Challenger proposes an expedition to discover the home of the pterosaur, but is dismissed by the science community. However, big game hunter Lord John Roxton, and Daily Gazette reporter Edward Malone both volunteer to join and finance the expedition. A sceptical, pompous Summerlee also joins. On the voyage to South America, Challenger reveals a map created by a Portuguese man named Father Luis Mendoz leading to a remote Brazilian plateau where he encountered dinosaurs he mistook for dragons during a previous expedition. They travel to a Christian mission in the Amazon, meeting Agnes Clooney and her uncle Reverend Theo Kerr, who condemns Charles Darwin's theory of evolution. Roxton immediately takes a liking to Agnes' tomboyish behaviour and flirts with her. Agnes volunteers to join the expedition as a translator. However, in the jungle, the expedition's porters flee out of superstition, but Kerr arrives, repeatedly trying to convince Challenger to turn back. They reach the edge of the plateau and find a cave concealing a pathway to the plateau but discover a blockage. They later find a gorge leading straight to the plateau, using a tree as a substitute bridge. However, when all but Kerr make it across, he suddenly knocks the tree into the gorge and leaves Challenger and the others stranded. Venturing in the plateau's jungle, they find Iguanodons and Hypsilophodons, a flock of Pteranodons, and a strange species of aggressive ape-men. Malone finds a lake which he names after his love, Gladys. Malone and Agnes are chased by an Allosaurus, but evade it when it falls into a man-made trap.