An American ship-builder named Bowen J. Tyler Jr. is rescued from the freezing waters off Greenland after his passenger liner is torpedoed by a German U-boat. Aboard the rescued lifeboat, he discovers a beautiful and refined English girl named Lys La Rue. Their survival becomes an immediate battle against cold, hunger, and relentless enemy fire from the submarine.
Events take a violent turn when the survivors, led by an intrepid English mate, manage to board and capture the very U-boat that sank them. However, the German commander Baron von Schoenvorts and his loyal crew remain prisoners, setting the stage for bitter internal conflict. Sabotage plagues the vessel as navigational instruments are systematically destroyed, and the submarine drifts thousands of miles off course into unknown southern waters.
Seeking fresh water and provisions, the crew navigates the submarine through a dark, subterranean river beneath towering cliffs and emerges into Caspak, a warm, hidden inland continent. This lost world teems with lush, unearthly flora and a terrifying menagerie of prehistoric monsters, including giant plesiosaurs, pterodactyls, and allosaurs. The crew establishes a fortified camp on the shore, attempting to tame an environment completely untouched by modern civilization.
Within Caspak, Bowen and his companions encounter various evolutionary strata of humanity, ranging from speechless brute-men and hatchet-wielding cave dwellers to more advanced spear-men. Each group maintains a primitive belief in spiritual ascension up the evolutionary ladder toward a higher race known as the Galus. Meanwhile, internal betrayals continue as the Prussian prisoners seize temporary control and abandon the allies.
Lys is abducted by a primitive cave-man, prompting Bowen to embark on a desperate, solitary rescue mission across the dangerous jungles and cliffs of Caspak. He tracks her through hostile tribes, confronts brutal native suitors, and eventually reunites with her. The novel concludes with the couple marooned on the barrier cliffs, choosing a life together in the lost world while dispatching a final manuscript in a thermos bottle into the sea.
