The book tells the adventures of five Americans on an uncharted island in the South Pacific. The story begins in the American Civil War, during the siege of Richmond, Virginia, the capital of the Confederate States of America. As famine and death ravage the city, five northern prisoners of war decide to escape by the unusual means of hijacking a balloon. The five are Cyrus Smith, a railroad engineer in the Union army; his black manservant Neb, who Verne repeatedly states is not a slave but an ex-slave who had been freed by Smith; the sailor Bonadventure Pencroff; his protégé Harbert Brown, a young boy whom Pencroff raises as his own after the death of his father; and the journalist Gedéon Spilett. The company is completed by Cyrus' dog 'Top'