A group of professional detectives challenge Nancy to tackle a mystery that they 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. Nancy follows her adversary to an outlaw's cave, and is threatened when she discovers an unusual treasure.