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 the buried treasure. The clue was a cipher carved on a stone tablet. Before the professors had time to translate the cipher, the tablet disappeared--and with it Dr. Pitt! Terry tells Nancy of his suspicions of the Tinos, a Mexican couple posing as scientists, who vanished the same night as Dr. Pitt.
The young detective is plunged into an adventure that demands all her ingenuity and bravery as she and her friends follow a tangled trail of clues that lead to the Florida Keys and finally to Mexico.