Keen young sailor Eunice Chantilly is thrilled when she is chosen to be one of the three Pollygog champions who will race to the famous Amon Line in the middle of the ocean. She never imagines that something colossal is lurking in the deep waters, waiting to be unleashed.

 When she limps back to Fiddyment Creek several days later, her boat is splintered and broken along with her spirit. Her hopes of becoming a shellback seem to be over.

 But one year later, a mysterious package arrives for Eunice which throws her whole world into chaos.  She and her best friend Kitty must embark on a voyage to find a long-lost treasure which is rooted in secrets powerful enough to change everything Eunice believes about her grandparents, her home, and most of all herself.