During an archaeological expedition in Mexico, two professors, the senior Dr. Joshua Pitt and young Terry Scott, find a clue to buried treasure.
The clue was a cipher carved on a stone tablet.
Before the older professor had time to translate it, he and the tablet disappeared!
Terry tells Nancy of his suspicions of a Mexican couple, posing as scientists, who vanished the same night as Dr. Pitt.
Nancy follows a tangled trail of clues that lead to Florida and Mexico and a secret of antiquity that can only be unlocked by three black keys.