Edward Malone, a reporter for the Daily Gazette, goes to his news editor, McArdle, to procure a dangerous and adventurous mission in order to impress the woman he loves, Gladys Hungerton. He is sent to interview Professor George Edward Challenger, who has assaulted four or five other journalists, to determine if his claims about his trip to South America are true. After assaulting Malone, Challenger reveals his discovery of dinosaurs in South America. Having been ridiculed for years, he invites Malone on a trip to prove his story, along with Professor Summerlee and Lord John Roxton, an adventurer who knows the Amazon and several years prior to the events of the book helped end slavery by robber barons in South America. They reach the plateau with the aid of Indian guides, who are superstitiously scared of the area. One of these Indians, Gomez, is the brother of a man that Roxton killed the last time he was in South America. When the expedition manages to get onto the plateau, Gomez destroys their bridge, trapping them. Their "devoted negro" Zambo remains at the base, but is unable to prevent the rest of the Indians from leaving. Deciding to investigate the lost world, they are attacked by pterodactyls in a swamp, and Roxton finds some blue clay in which he takes a great interest. After exploring the plateau and having some adventures in which the expedition narrowly escapes being killed by dinosaurs, Challenger, Summerlee, and Roxton are captured by a race of ape-men. While in the ape-men's village, they find out that there is also a tribe of humans (calling themselves Accala) inhabiting the other side of the plateau, with whom the ape-men (called Doda by the Accala) are at war. Roxton manages to escape and team up with Malone to mount a rescue. They arrive just in time to prevent the execution of one of the professors and several other humans, including a young man who turns out to be a prince of sorts among the Accala. The rescued Accala then take the party to their tribe. With the help of the expedition's firepower, the Accala defeat the ape-men, taking control of the whole plateau.