The film begins with a bottle with a written manuscript inside it being thrown into the sea, hoping for it to be discovered later. Floating to the coast of England, a sailor discovers the bottle and opens to read the manuscript as Bowen Tyler (Doug McClure) narrates the events, much as we see in many of Edgar Rice Burroughs' novels. The story is set during World War I and involves the survivors of the sinking of a British merchant ship who are taken on board a German U-boat. Bowen Tyler and Lisa Clayton (Susan Penhaligon) are passengers on the ship torpedoed by Captain von Schoenvorts (John McEnery). Along with a few surviving British officers, Tyler convinces the other men to take over the surfacing submarine, this being their only chance for survival. After they confront the Germans on the deck, a fight ensues and they seize the German U-boat. Tyler takes command, hoping to sail to a British port. Von Schoenvorts has his crew steer toward a safe sea port, but German officer Dietz (Anthony Ainley) breaks loose and smashes the sub's radio. Off course and running out of fuel in the South Atlantic, the U-boat and its crew happen across an uncharted subcontinent called Caprona, a fantastical land of lush vegetation where dinosaurs still roam, co-existing with primitive man. There are also reserves of oil which, if the Germans and British work together, can be refined and enable their escape from the island. Tyler and von Schoenvorts agree to work together toward that end. Tyler discovers the secret of Caprona: individuals evolve not through natural selection, but by migrating northward across the island. With the submarine working again, a sudden outbreak of volcanic eruptions occurs across the island. Dietz starts a mutiny and shoots Captain von Schoenvorts to take command. He abandons Tyler and Clayton in Caprona in an attempt to escape, but the U-boat cannot function in the boiling waters and the crew is killed as it sinks. Tyler and Clayton are stranded, and be