Prompted by the concerns of a young child, Nancy investigates a small studio on the Chatham estate. She discovers that there is a connection between the mysterious occurrences at Ship Cottage and her search for a treasure island. With only half of a map, Nancy sets out to find the man who possesses the rest of the map that will pinpoint the location of buried treasure. Constantly beset by danger and intrigue, Nancy courageously outwits her enemies and solves one of the most challenging cases in her career as a teenage investigator.