When a balloon escaping from Richmond, Virginia, during the Civil War, is blown off course, comes down on the Pacific Ocean. One of its occupants, Cyrus Smith, falls or jumps off into the sea with his dog. The other occupants, three men and a boy, jump down on a beach. Cyrus Smith manages to make it to shore but has no idea how he has reached a cave half a mile from the sea. Other mysterious things happen on the uninhabited Island. After the men have made a list of things they need, a large box appears on the beach and inside are all the things they have listed. When pirates come to disturb them in their new cave house, their ship is torpedoed and sent to the bottom. Finally, when the boy, Herbert, falls sick, a medicine appears by his bed and so he is saved. But their mysterious helper suddenly needs help of his own. He tells them how to find him – at the bottom of the deep well in their cave house. He is no other than Captain Nemo, anti-hero of another Jules Verne story, Twenty Thousand Leagues Under the Sea. He is at the end of his life and he wants the men to take his riches and scuttle his submarine, Nautilus, after his death, with him on board. The friends spend a little longer on the island but are eventually rescued by a British survey ship, free at last to start or return to families, and write or tell of their adventures and their mysterious helper.