Professional detectives challenge Nancy to tackle a mystery. The young sleuth is tasked find an important message hidden by a missionary centuries earlier in a hollow oak tree in Illinois. While searching the woods for the ancient tree, Nancy and her friends stay 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 girl investigator pursues her dangerous adversary to an outlaws’ cave and is threatened when she discovers an unusual treasure.