The plot focuses on the adventures of five Americans on an uncharted island in the South Pacific. During the American Civil War, five northern prisoners of war decide to escape, during the siege of Richmond, Virginia, by hijacking a balloon. During their stay on the island, the group endures bad weather, and domesticates an orangutan, Jupiter. There is a mystery on the island in the form of an unseen deus ex machina, responsible for Cyrus' survival after falling from the balloon, the mysterious rescue of Top from a dugong, the appearance of a box of equipment (guns and ammunition, tools, etc.), and other seemingly inexplicable occurrences…