After their parents, the rulers of the Star Lands, are deposed and their kingdom flooded, Asa and Rollo hide out until they come into possession of a ghost ship and magic powers, which they use to find the baby sister they thought had died with their parents.
When underwater pirates steal Asa's Moonstone Star, she and her brother Rollo search for it across their vast, flooded kingdom, and along the way face ghosts, sea giants, and secrets about their own past.
