In the best tradition of classic young adult mystery novels, Gonzalo and Vicky try to solve the problem posed by a piece of old letter found inside a copy of Don Quixote. Although they have very few data, little by little they unravel the enigma and learn that it has to do with a mysterious object which "could become the scourge of humanity." The two young people must confront the dangerous guardians of the object during the last days of Franco.