During the German occupation of Rome in 1943 an athletic Irish priest, Monsignor Hugh O'Flaherty (Gregory Peck), devotes all the time he can spare from his work at the Vatican to hiding Allied POWs from the Nazis.
Col. Herbert Kappler (Christopher Plummer), Rome's chief Gestapo Officer, suspects O'Flaherty of hiding escapees but can do little about it because of the priest's Vatican diplomatic immunity. But when he unearths proof of O'Flaherty's complicity, he orders that the priest be captured or killed if he is seen outside the Vatican walls.