The true story of Monsignor Hugh O'Flaherty (Gregory Peck), a courageous Irish priest working in the Vatican during the German occupation. O'Flaherty devotes all his time and energy to hiding refugees and Allied POWs from the Nazis, building a network of hundreds of people to help him with his efforts.

Colonel Kappler (Christopher Plummer), the local Gestapo chief, learns of O'Flaherty's activities. The priest has diplomatic immunity because of his Vatican post, but Kappler orders that he be captured or killed if seen outside the Vatican walls.

Pope Pius XII remains aloof insisting on the church's neutrality. Working closely with a brave widow of an aristocrat, O'Flaherty uses disguises to slip in and out of the Vatican, continuing his dangerous mission until Rome is liberated, and saving thousands of innocent people from death.