The movie is based on the 1918 Edgar Rice Burroughs story and is set during the World War I era. I find it interesting that Burroughs' tales start out in one milieu before transitioning into science fiction fantasy; another example would be "John Carter From Mars" which opened with a narrator during the time period of the Civil War. In this tale, Burroughs opines on a lost world he calls Caprona, uncharted and forgotten for two hundred years and set somewhere within the hidden confines of the Antaractic.
The adventure begins with the sinking of the twenty thousand ton British freighter Montrose by a German submarine. Survivors of the Montrose manage to find their way aboard the sub as it surfaces and in a highly unlikely maneuver, manage to subdue the German Captain and his crew. Further suspension of disbelief is required as the Brits, along with American Bowen Tyler (Doug McClure) trade control of the sub with the German Von Schoenvorts (John McEnery) once again before they decide they need to work together as their rations and water supply are at minimum, particularly after Tyler's maneuver of torpedoing a German supply ship that was to rendezvous with the submarine. I thought it pretty convenient that Tyler's father had a hand in designing submarines, which in turn gave him an advantage in knowing the sub's machinery and operating mechanisms. What were the odds?
Things pick up once the seamen make it through an underwater tunnel that brings them to the tropical land of Caprona. Encountering a Neanderthal type being that becomes sort of a guide, the adventurers run across a variety of dinosaurs from earth's ancient past, including pterodactyls, a diplodocus, plesiosaurus and a couple of allosaurs that they summarily shoot to death. I always wondered in movies like this why the principals begin firing at creatures that aren't posing any particular danger. Perhaps in an attempt to elicit the viewer's sympathy in this regard, another encounter with a styracosaurus ends with the dying creature shedding a tear.
The story line offers an interesting concept for one's consideration when biologist Lisa Clayton (Susan Penhaligon) proposes that the land they've encountered is home to animals and human-like beings that represent the various stages of evolution that Earth had gone through. Surmising that if they continue traveling in a northerly direction, they would eventually come across more advanced creatures, the explorers are waylaid in their quest by warring sub-humans and a seismically active environment that leads to a fiery finale. Volcanoes explode and fissures open in the ground that emit fire due to the presence of burning oil.
The story doesn't have a typical happy ending for the principal characters, Tyler and Clayton. Left stranded on purpose by a vengeful German who opposes his Captain, the pair can only look on in astonishment as the submarine is hijacked, but their amazement is tempered when the sub is destroyed during the volcanic eruptions and attendant upheaval. The picture hints an Adam and Eve type of new beginning for them, now left to their own devices to make the best of an unplanned and unimagined future together.
Review by classicsoncall from the Internet Movie Database.