The Clue of the Black Keys (The Nancy Drew Mysteries No. 28)

Terry Scott, a young archaeology professor, seeks Nancy’s help in unearthing a secret of antiquity which can only be unlocked by three black keys. While on an archaeological expedition in Mexico, Terry and Dr. Joshua Pitt came across a clue to buried treasure. The clue was a cipher carved on a stone tablet. Before the professor had time to translate the cipher, the tablet disappeared – along with Dr. Pitt!