Danger follows Kay as she searches for a priceless violin, investigating the theft of a priceless Stradivarius violin from her friend Rosa. Kay uncovers a mystery involving a mysterious key, a dishonest antique dealer, and her rival, Ethel Eaton.
Kay finds her friend Rosa bound and gagged, with her father's valuable violin stolen. Kay discovers a key with the number "777," leading her through various adventures and dangers. She faces dangerous situations, including being locked in a closet in a burning house, following clues to solve the case.
This series of mysteries was just as popular as Nancy Drew in its day, some even believing it to be superior.