Why was Billy Mack summoned to Rome on the next plane? Billy thought it was to experience an historic event, but quickly learned otherwise. After a succession of over 266 popes dating back to St. Peter, a sede vacante, a papal vacancy, was not an unusual historical occurrence for the Vatican but a call from Cardinal Kauftheil suggested that this vacancy was unlike any other. The Pope's mysterious death was only a prelude to what was about to confront the Catholic Church. As the College of Cardinals went about its duty to elect a new Pope, it needed a guardian angel and Billy was 'requested' to assume that role, a role he could not refuse. In order to fulfill his duty there was only one rule - The Cardinal Rule - requiring Billy to take what he discovers to his grave. When he learns the world's best-kept secret, a bright red target is inevitably painted on his back. That is not the only secret Billy learns. On the run with Sister Angela, a nun hiding a mysterious past, the two must outwit the ultimate of all evils, and save the Church from the Apocalypse -- an apocalypse coming from within the walls of the Vatican.