Nancy investigates a small ship cottage at the Chatham estate and discovers a connection between the mysterious occurrences at the cottage and an island where a lost treasure is said to be buried. With one half of a map, Nancy sets out to find a missing twin brother who holds the other half.
Nancy explores a tiny cottage on the Chatham estate, uncovering a connection between its strange happenings and a mythical island where a hidden treasure is said to lie.  She embarks on a quest to locate her missing twin brother, who possesses the other half of a map leading to the treasure.