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! Terry tells Nancy of his suspicions of the Tinos, a Mexican couple posing as scientists who vanished the same night as Dr. Pitt. Nancy and her friends follow a tangled trail of clues that lead to the Florida Keys and finally to Mexico in this suspense-filled story that will thrill readers.