A fifteen-year-old orphan living with her aunt, uncle and cousins leaves their New England home when financial and health troubles force a move to a secluded island. Over the course of a year she leads explorations with her cousins, discovers caves and signs of pirate activity, and uncovers an old map associated with Sir Henry Morgan. When the map is stolen the group pursues clues through lagoons, inlets and inland trails, confronts unexpected visitors and the return of a sea captain, and ultimately locates a hidden, long-lost treasure.