Why are the Bowens - a missionary couple who recently returned to the United States - having so much trouble finding their missing seventeen-year-old granddaughter, Joanie Horton? Nancy and her friends travel to Deep River, the town where the young Jonie lived. From the motel where Nancy and her friends stay, they can see an intriguing castlelike structure with a drawbridge. Gossipy Mrs. Hemstead at the village tearoom insists that Moonstone Castle is haunted. Curious, the three girls attempt to explore the abandoned castle, but an ominous voice warns them away.

Other strange happenings in Deep River convince Nancy that there is a connection between Moonstone Castle and the mysterious moonstone gift. But what is the significance? And where does the baffling disappearance of Joanie Horton fit into the intricate puzzle?