The Seagrave family are on a stout ship en route for Australia. In a storm the ship is struck by lightning, set on fire, and dismasted. The crew make off in the ship's boats and the family are abandoned on board the sinking ship, along with an elderly seaman, Masterman Ready. But she doesn't sink, instead drifting to the shelter of an uninhabited island, where they scramble ashore, and set up a habitation. How they get on, what ordeals they have to endure, and how they are eventually rescued, is the substance of the book.