Based on a novel by Arthur Conan Doyle, a scientist and an explorer's daughter lead a rescue party back to the Amazon jungle, where dinosaurs are living.
A newspaper sends Edward Malone (Lloyd Hughes) to cover a lecture by Professor Challenger (Wallace Beery), who has banned journalists for not believing his discovery of living dinosaurs. Edward gets in with big-game hunter John Roxton (Lewis Stone). Challenger asks for volunteers to go with him to see the dinosaurs living in the jungle. When Edward admits he is a reporter, Challenger chases him out. Edward follows him home and climbs in his window. Edward tells Challenger that he must go on the expedition because his fiancée won't marry him until he risks his life. Challenger attacks Edward, and the police break it up. Mrs. Challenger (Margaret McWade) criticizes her husband for thrashing those who are trying to help him. Edward says that he is a friend of Roxton, and Challenger accepts him. Roxton comes in, and Challenger shows Edward a diary that belonged to the explorer Maple White, who was left behind. His daughter Paula White (Bessie Love) wants to go back to find her father. Edward says that his paper might sponsor a rescue party.
The party travels in canoes in a secret river. They see a pterodactyl flying. They chop a tree down to cross a chasm. They see a brontosaurus, and Challenger says it eats only plants. The brontosaurus causes the tree bridge to fall, and they are trapped on the plateau. They watch a predatory dinosaur attacking other dinosaurs. They shoot at one and see an ape man (Bull Montana). Challenger bends a tree to make a catapult. Their men below are making a ladder. Roxton is in love with Paula, but he sees Edward kissing her. They announce their intention to marry. The predator pushes the brontosaurus off the cliff into the river. A fire spreads, but Challenger escapes.
The monkey Jocko takes the ladder up the cliff to Paula, and she is the first to climb down. Then the ape man interferes and is shot. Major Hibbard (Charles Wellesley) arrives and offers to help transport the brontosaurus so they can ship it to London.
Challenger lectures to a large audience and says that Edward is supervising the unloading of the brontosaurus. Edward calls Challenger and says the brontosaurus got loose. People in the streets see it and panic. The brontosaurus knocks over lamp posts and damages a building. When it causes the London bridge to collapse, it falls into the river. In the final scene Roxton congratulates Edward and Paula.
This science-fiction adventure reflects the human curiosity
about the enormous dinosaurs and the little-known Amazon jungle.