While on a vacation in Venice with his family, Gabriel Allon learns that Pope Paul VII has died of a heart attack. Then he is contacted by the deceased's private secretary, Archbishop Donati and told there is suspicions over the death. One, the Swiss Guard protecting the pontiff that night has disappeared. Two. the letter His Holiness was writing is missing. That letter was addressed to Allon and mentioned a book the man had found in the archives which he had called "most remarkable".