Maverick scientist Prof. Challenger (Wallace Beery) claims that dinosaurs still exist on a remote Brazilian plateau, and to prove his assertion he leads an expedition up the Amazon. The team indeed finds the evidence they seek when one of the beasties cuts off their only route back to civilization. Barely escaping with their lives, the party returns to London -- live brontosaur in tow. Sure enough, the big bruiser gets loose, and bedlam ensues.