On a flight to the United States of America, members of a terrorist cell calling themselves the "Muslim Revolutionary Front" take control of Pope Pius XIII's plane - Shepherd One. On board is a nuclear bomb, half the size of the one that destroyed Hiroshima. Their demand to the US government is that the five top leaders of Mossad be executed. While the US government feels compelled to comply with the demands, risking the intelligence network among the Allies, Pope Pius XIII is not entirely defenseless. For on board Shepherd One is his personal guardian angel, a man who is also the leader of a secret papal elite command - the Knights of the Vatican. Their mission is simple: to regain control of the plane and eliminate all opponents.