After a wartime sea attack a survivor recovers a sealed manuscript and narrates the ordeal: picked up by a submarine and carried to a hidden, primitive island where long-extinct creatures and archaic human societies persist. The account follows the group's hazardous landings and expeditions, vivid encounters with prehistoric fauna, rivalries and moral dilemmas among survivors, improvisations for shelter and food, and repeated attempts to secure rescue or escape. The narrative blends adventure, speculative natural history, and survival drama framed by the recovered-document device.