An antique dealer's revelation about a former queen's priceless heirloom starts Nancy on a trail of exciting adventures.

Madame Alexandra, now living incognito in River Heights, asks Nancy to find her missing grandson. With only one clue to go on — a faded photograph of the prince at the age of four — the young detective begins her search, which rapidly involves her in a series of dangerous and harrowing adventures. When Nancy needs help, she calls on Bess, George, Burt, Dave, and her special friend, Ned. How Nancy's discovery of the heirloom's secret unmasks a slick imposter and reunites the long-separated family climaxes this suspense-filled mystery story.