Aunt Eloise Drew invited Nancy and her friends to Mirror Bay Bide-a-Wee cabin near Cooperstown, New York, for a visit and a chance to solve the mystery of the woman who glides across the water. Upon their arrival, Nancy becomes mixed up in a vacation hoax and is nearly arrested for fraud.

On the wooded mountain near the cabin, further exciting events await Nancy and the other girls. In the deep forest, a weird luminescent green sorcerer appears who threatens to cast an evil spell on anyone investigating his strange activities. In a dangerous twist of circumstances, Nancy finds that solving one mystery helps to solve another. What happens when the young detective and her friends uncover a cleverly concealed criminal operation?