Edward Malone, a young reporter for the Daily Gazette, asks his editor for a dangerous assignment to impress the woman he loves, Gladys, who wishes for a great man capable of brave deeds and actions. His task is to approach the notorious Professor Challenger, who dislikes the popular press intensely and physically assaults intrusive journalists. The subject is to be his recent South American expedition which, surrounded by controversy, guarantees a hostile reaction. As a direct approach would be instantly rebuffed, Malone instead masquerades as an earnest student. On meeting the professor he is startled by his overwhelming intellect - his intimidating physique too - but believes his ruse is succeeding. Seeing through the masquerade, then confirming Malone's scientific knowledge is non-existent, Challenger erupts in anger and forcibly throws him out. Malone earns his respect by refusing to press charges with a policeman who saw his violent ejection into the street. Challenger ushers him back inside and, extracting promises of confidentiality, eventually reveals he has discovered living dinosaurs in South America. At a tumultuous public meeting in which Challenger experiences further ridicule, Malone volunteers for an expedition to verify the discoveries. His companions are to be Professor Summerlee, and Lord John Roxton, an adventurer who helped end slavery on the Amazon; the notches on his rifle showing how many slavers he killed doing so.Running the gauntlet of hostile tribes, the expedition finally reaches the lost world with the aid of Indian guides, who are superstitiously scared of the area. When they finally succeed in scaling the cliffs and entering the lost world another helper, Gomez, whose brother was killed by Roxton in his fight against slavery, destroys their makeshift bridge. The four explorers are now trapped on the plateau.Whilst investigating the wonders of the lost world, discovering many plants and creatures thought to be extinct, they narrowly escape an attack from pterodactyls. Although barely escaping with their lives, Roxton takes great interest in nearby blue clay deposits. At night a ferocious dinosaur is about to break through the thorn bushes surrounding their camp; Roxton averts disaster by bravely dashing at it, thrusting a blazing torch at its face to scare it away. Later, all except Malone are captured by a race of ape-men. Whilst in captivity they discover that a tribe of natives, with whom the ape-men are at war, inhabit the other side of the plateau. Roxton escapes and together with Malone mounts a rescue, preventing many unpleasant deaths, including a young native who is a prince of his tribe. The rescued natives take the party to their village, then with the help of their firepower return to defeat the ape-men. After witnessing the power of their guns, the tribe wish to keep them on the plateau but, helped by the young prince they saved, they eventually discover a tunnel leading back to the outside world.Upon return to England, despite full reports from Malone many detractors continue to dismiss the expedition's account, much as they had Challenger's original story. Anticipating this, at a public meeting Challenger produces a young pterodactyl as proof, transfixing the audience and leaving them in no doubt of the truth. The explorers are instantly feted as heroes, and on a wave of adulation find themselves carried shoulder-high from the hall by cheering crowds.At a private celebratory dinner, Roxton reveals to the others that the blue clay contained diamonds, and that he managed to extract about GBP200,000 worth, which is to be split between them. Malone returns to his love, Gladys, hoping she will recognise his achievements. Instead, he finds she has now changed her mind and married a very ordinary man instead, an insignificant clerk. Astonished at this turn of events, and with nothing to keep him in London, he decides to accompany Roxton back to the lost world.