Professor Challenger (Claude Rains) reports to the London Zoological Society that he has discovered living dinosaurs on an expedition to a hidden plateau in the Amazon rain forest. The Society is sceptical to say the least, so he organizes a return expedition. Included, to his chagrin in some cases, are big-game hunter Lord John Roxton (Michael Rennie), rival scientist Professor Summerlee (Richard Haydn), newsman Ed Malone (David Hedison ) whose publisher is financing the expedition, and the publisher’s adventurous daughter Jennifer (Jill St. John) and son David ( Ray Stricklyn). In the jungle, they meet up with helicopter pilot Manuel (Fernando Lamas), with a sidekick named José Costa (Jay Novelo).

During the first night on the plateau, a dinosaur wrecks the helicopter. Malone chases a primitive jungle girl (Vitina Marcus) through the web of a giant spider. The native girl falls for David, Roxton argues with most everybody, and Malone and Roxton get into a fistfight over Jennifer. They discover the diary of a lost explorer, Burton White (Ian Wolfe), who mentions Roxton. He had been there before and says there is a fortune in diamonds, which Costa finds tempting.

Malone and Jennifer barely survive a battle between lizards in saurian drag, cannibals capture some of the party but the native girl leads them to safety. They find Burton, now living as a blind hermit. Then they are pursued by the cannibals again, attacked by spider plants, and encounter a dinosaur in a lava pit guarding the diamonds which kills Costa and Gomez as the latter breaks a rock dam, killing the dinosaur. The volcano erupts—some days are like that—and the party escapes from the plateau, Challenger carrying the egg of a T-Rex. Roxton’s pockets are full of diamonds and he shares them with everyone. Challenger drops the egg and the baby T-Rex hatches, so Challenger decides to take it back to London with him.

The color Cinemascope picture was directed by Irwin Allen, based more or less on the book by Arthur Conan Doyle, though so little of the book survives that IMDB refused to list him as a writer. The cast is highly watchable and the movie is full of action and adventure, but the dinosaurs leave much to be desired. You must remember that Irwin Allen is the guy who thought submarines could be threatened by sinking icebergs. Willis O’Brien was hired to do the creatures, but in the end they were only lizards, iguanas, and crocodiles fitted with miniature horns and fins because they didn’t have the budget for stop-motion. Also, there was a lot of stock footage from Allen’s TV series—Land of the Giants, Lost in Space, The Time Tunnel, and Voyage to the Bottom of the Sea

Willis O’Brien made hundreds of sketches but none of his ideas were used. The budget of this and other 20th Century Fox films were slashed because costs for Cleopatra (1963) were spinning out of control, but it must be admitted that Cleopatra saved the studio’s bacon. Vitina Marcus came back as the native girl in Voyage to the Bottom of the Sea. Some of the sets were originally built for Journey to the Center of the Earth (1959). The origin of Doyle’s lost world was the Mount Roraima and other high tepuis of the Venezuelan highlands. They are indeed evolutionarily isolated.