Terry Scott, a young archaeology professor, seeks Nancy's help in unearthing a secret of antiquity which can be unlocked by three black keys. While on an archaeological expedition in Mexico, terry and Dr. Joshua Pitt came across a clue to the buried treasure. The clue was a cipher carved on a stone tablet.
Nancy is asked to solve a puzzling mystery then encounters a second case. The first concerns a valuable collection of rare books. Wealthy Mrs. Merriam has commissioned a supposedly reputable art dealer to sell it, but soon believes the man is a swindler. The second mystery involves the theft of a beautiful marble statue, rumored to whisper, has an uncanny resemblance to Nancy! As suddenly as it disappeared, the marble statue is returned and is discovered to be a reproduction! To solve both mysteries, Nancy adopts an alias and a disguise, becomes an employee of the suspicious dealer, gathering evidence against a clever ring of art thieves. This book is the revised text. The plot of the original story (©1937) is different.