Young Gillbert is the prince of the underwater world of Atlanticus. Together with his best friend Sherbert and mysterious new friend Anne Phibian, Gillbert is curious to where all these bottled messages floating throughout the ocean are coming from. Could the fiery Pyrockians from space have something to do with it? Or maybe it is something more mysteriously monstrous? Plus, Gillbert must deal with being a new big brother, when his little sister Matilda is actually much, much bigger than him!