During the American Civil War, a group of Union prisoners of war manage to escape in a hot air balloon and find themselves stranded on a mysterious island in the South Pacific, filled with enormous plants and creatures. Forced to rely on their resourcefulness to stay alive and come up with a plan to get back home, they face numerous challenges. This adventure is a follow-up to the famous story '20,000 Leagues Under the Sea'. In the year 1865, amidst the chaos of Richmond's siege in Virginia, Union POW Captain Harding, along with Neb and Herbert, flee in a balloon during a fierce storm, accompanied by two Confederate captives - Sergeant Pencroft and journalist Spilett. Driven by the wind, the balloon lands on a remote island in the South Pacific near New Zealand, where Captain Harding assumes leadership. Struggling to find food, they realize they are marooned on the island, where they encounter giant creatures like a massive crab. As they build shelter and explore the island, they stumble upon two shipwreck survivors, Lady Mary Fairchild and her attractive niece Elena. Their discovery of a trunk containing weapons and navigation tools, along with a journal left by a previous castaway, uncovers the island's mysteries. When pirates arrive and a volcanic eruption threatens their safety, they receive unexpected help from Captain Nemo of the Nautilus submarine, believed to have sunk off the Mexican coast years ago. Nemo, known for his attempts to promote peace, aids them in escaping the island before it's too late.