A group of professional detectives challenge Nancy to tackle a mystery that they have failed to solve: find an invaluable message hidden by a missionary centuries ago in a hollow oak tree in Illinois. While searching the woods for the ancient tree, Nancy and her friends live with a group of young archaeologists who are excavating prehistoric Indian burial mounds on a nearby farm. A shadowy enemy stalks Nancy and harasses everyone at the dig. The young investigator pursues her dangerous adversary to an outlaws' cave, and is threatened when she discovers an unusual treasure. How Nancy, with few clues to go on, solves this complex mystery will thrill all readers.
Carolyn Keene is a pen name used by a variety of authors for the classic Nancy Drew Mystery series. The first author to use the pseudonym was Mildred Wirt Benson, who wrote 23 of the original 30 books. Other writers who have adapted the "Carolyn Keene" moniker include Leslie McFarlane, James Duncan Lawrence, Walter Karig, and Nancy Axelrod.