Fernando de Zúñiga Ayala was born in Madrid in 1634, in a family of Basque origin. The illegitimate child of Francisco de Zúñiga, VII Count of Miranda, and of Inés Ayala, midwife of the court of Philip IV, he moved to Salamanca when he was fifteen to study medicine. In 1659 he married Pilar Maldonado, with whom he had two daughters, Cristina and Leonor, and who would die three years later.

This event conditions his character and his interest in the occult. His friendship with queen regent Mariana of Austria leads him to watch over the delicate health of her son, King Charles II. In 1669 he is awarded the Viscountcy of Castañar and he starts to provide sporadic services to the Grand Inquisitor Don Diego Sarmiento de Valladares. The 1680 Auto de Fe seriously affects his conscience: he stops working for the Holy Office and goes back to practicing medicine in Salamanca.

He occasionally investigates strange events that his knowledge, together with his common sense seasoned with intuition —as he likes to define it—, allows him to discover. In his first case he dealt with mysterious deaths whose victims ended up being turned into crucified figures. Almost one year later, he must find out the causes behind the death of his friend Pedro Urtiaga, poisoned after a game of cards … But the adventures of Don Fernando de Zúñiga have only just begun.