Living in the ordinary world is anything but ordinary for twins Percy and Nell Shearwater. After all, Percy is not like most eleven-year-old boys. Most eleven-year-old boys don't have gills. When the grip of death tightens its hold on the twins' mother, Percy resolves to do whatever it takes to find a cure. But what if that means embarking on a secret and treacherous journey through uncharted waters, to the island of Doona... an island which some say doesn't even exist? While Percy confronts the perils of the sea to save his dying mother, Nell will need to rely on her own mysterious gift if she is to have any chance of seeing her brother again. There's just one thing that lies in her way: the Curse of Dearmad...