Clue in the Ancient Disguise — Carolyn Keene

A ROYAL BLAST FROM THE PAST
The legacy of the French Revolution lives on in River Heights! Pierre Michaud, a handsome young Frenchman, has stumbled onto an ancient family mystery, and Nancy is convinced that the secret holds the key to a string of burglaries at the local museum.

The truth lies hidden deep in the darkest vaults of the museum, where Nancy comes face-to-face with a ghastly figure out of Pierre's past. She unravels a fabulous tale of noble romance and aristocratic intrigue, and makes a startling discovery that could change Pierre's life forever!