In 1943, with Rome occupied by the Nazis, Monsignor Hugh O'Flaherty (Gregory Peck) does all he can to help Allied POWs, providing them with hiding places and assisting them in their flight from the Germans. Gestapo Chief Colonel Herbert Kappler (Christopher Plummer) grows suspicious of O'Flaherty, but is frustrated by the priest's diplomatic immunity. Finally, he issues an order that O'Flaherty either be captured or shot if he is ever seen outside the walls of the Vatican.