Carson Drew foreshadows his daughter's future adventure when he says, half-jokingly, "Nancy, don't let any thief at the party ruin your holiday." A masquerade party at the Hendrick mansion quickly turns into a mystery when Nancy's favorite young man , Ned Nickerson, track a stranger climbing a flower trellis to the second floor. Who is this mysterious man in a black cloak and an exotic woman in a Javanese costume? Could they be members of a wily gang of thieves who infiltrate wealthy people's parties and steal jewelry and artwork? And why is the owner of the black velvet hooded mask Ned finds in the Hendricks' garden so desperate to get it back? To find the answers to these questions, Nancy and her friend Georgie Fain devise a daring plan. The girls change places! Georgie soon discovers that while playing amateur detective is fun, it can be dangerous to impersonate Nancy Drew.