The murder of an elderly French traveler is only the first of a chain of mysterious crimes taking place in a quiet inn in Rome.

The famous diplomat and intelligence officer of Louis XIV, Abbé Melanie and his “Doctor Watson” (or, more precisely, “Abbot Adson”) - the cook of the inn.

The pursuit of the mysterious killer leads them further and further - either into the dark underground world of the Roman catacombs, then into the alchemical laboratories, then into the labyrinths of the Vatican, where an assassination attempt is being prepared the life of Pope Innocent XI...

However, the mystery still remains a mystery, and the criminal is still one step ahead of the hunters!