This book is a sequel to "Twenty Thousand Leagues Under The Sea". A party of British adventurers, who had been ballooning, but whose trip had ended by being cast away on a Pacific island, have various setbacks due to both pirates and convicts who had escaped from jails in mainland Australasia. They realise that at times there appears to be some kind of entity that is looking after them. This entity proves to be none other than Captain Nemo whom the reader is expected to have met before with his submarine "Nautilus" in "20,000 Leagues". Captain Nemo has been living in a huge cave inside the very volcanic island, where he is surrounded with immense wealth. But he is nearing the end of his life. We are present at his end. But what happens after that is of great interest.