The Clue of the Black Keys, Nancy Drew Mystery Stories, number 28 in the series. Author: Carolyn Keene. Hardcover, 214 pages. Publisher: Grosset & Dunlap, no 9528, copyright 1951 and published in 1962.

Synopsis: In The Clue of the Black Keys, a young archaeology professor named Terry Scott seeks Nancy Drew's help to solve an ancient mystery. While on an expedition in Mexico with Dr. Joshua Pitt, the two professors discovered a cipher carved on a stone tablet that contained a clue to buried treasure. Before they could translate the cipher, the tablet and Dr. Pitt mysteriously disappeared.

Terry suspects a Mexican couple, the Tinos, who were posing as scientists and vanished the same night as Dr. Pitt. Nancy and her friends follow a series of clues that lead them from River Heights to the Florida Keys and finally to Mexico. The secret to the ancient treasure can only be unlocked by three black keys, and Nancy's investigation involves dodging danger and uncovering the truth behind the stolen artifacts and the missing professor.

Condition: The front cover has wear on the bottom right corner tip (see photos).  The spine, binding, and inner pages are all in very good condition.