Professor Challenger (Wallace Beery), an eccentric scientist with a big anger management problem, starts an expedition into the depth of the Amazon basin to proof his theory of the survival of some dinosaur species on a difficult to reach plateau.
After he and his companions reach this Lost World and survive some adventures, they are able to return to their home in London, bringing a mighty big Brontosaur with them. The dinosaur turns out to be a true pioneer and starts a mild monster rampage.
This first screen adaptation of Sir Arthur Conan Doyle's book is mainly of historical interest as a relatively early work of stop-motion pioneer Willis O'Brien (eight years before King Kong). If you just fast forward through the human interest parts to the dinosaur footage, you won't really miss anything of interest. And even the effects are not completely up to O'Brien's standards.
In this The Lost World shows itself to be a precursor of the modern blockbuster, which also deals in a combination of sentimentality and spectacle.
But hey, at least there is a rampaging giant monster.