In 1934, the researcher Maple White and his assistant Azbek, discover a world populated by dinosaurs situated on a Mongolianplateau. Soon after the pair are attacked by Eudimorphodons who kill Azbek, though White survives.
White seeks out his friend, George Challenger, giving an account of his adventures, proposing a further expedition to fully explore the plateau.
In London, Challenger organises a lecture in an attempt to convince his colleagues, Professor Summerlee and Lord Thomas, to finance the further expedition with the goal of proving dinosaurs exist on the plateau. Lord Thomas grants permission for the expedition on the condition Summerlee accompanies Challenger. Oscar Perreault, a spectator at the lecture, proposes to pay for the expedition's expenses if the scientists capture a living dinosaur and sends his associate, John Roxton, to represent his interests.
Challenger also invites Arthur Malone (a journalist) and Maple White's daughter Amanda, on the expedition as well.
In Mongolia, the team meets their guides, Myar and Djena. En route, their transportation suffers a breakdown and the journey continues on foot, after which one of the guides is killed by a prehistoric insect.
On reaching the foot of the plateau, Amanda is abducted by Neanderthals, prompting a search. Challenger and Roxton find Amanda suspended from a ritual framework. During the rescue, they face the Neanderthals, who they quickly defeat. The team attempts to escape the plateau in a hot air balloon. Summerlee observes nearby Quetzalcoatlus, and they attack the balloon, causing Myar to fall. The balloon also suffers a large tear and drops back to the plateau.
They discover symbols on a cave wall, portraying a tribe, the Kerraks, who interacted with prehistoric creatures and fought the Neanderthals. The Neanderthals eventually defeating the Kerraks to avoid the destruction of the 'Lost World'.
A second larger cave is then discovered, where Roxton attempts to explode a dynamite charge to kill the team and escape alone; he is foiled by Challenger, however.
An escape attempt is then made by fashioning a parachute using the remains of the balloon and after suffering a further attack by the creatures which inhabit the plateau, they manage to escape.
Returning to London, Challenger communicates the death of his companions to the students and, in an attempt to discourage further expeditions, he chooses to state his expedition found nothing.
In the last scenes of the film, it is revealed Malone has survived and adapted to life on the plateau.
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