An American fugitive flees to Rome and tries to elude capture by masquerading as a priest.

Father John X. Halligan is a Catholic priest visiting Rome for the 1950 Holy Year. On the long voyage from New York City to Genoa, he makes friends with Joe Brewster, his cabin mate. Unknown to Halligan, Brewster is a career criminal wanted by American authorities; he faces a life sentence. The ship docks in Genoa. When he sees police waiting, Brewster steals Halligan's clothing, cassock, hat and passport in order to evade arrest. Two priests appear to welcome "Father Halligan". When Halligan disembarks, wearing Brewster's flamboyant clothes, he is arrested. The Genoa commissario of police believes his story when he chants a portion of the mass. Meanwhile, Brewster makes friends with an Irish priest and ends up staying with him in Rome at the Monastery of the Three Saints. At a concert, he remembers his days as a choir boy. Now dressed in borrowed clothes, Halligan reluctantly promises to aid the police. In Rome, he meets the cynical commissario of police. On the way to headquarters, they stop for a procession. Halligan sees Brewster in it and says nothing. The commissario tells Halligan that he will meet him the next morning at the Monastery of the Three Angels, where he is registered. Halligan, who has no place to stay, realizes that this must be Brewster and finds him there. Brewster asks for just one day. Halligan agrees-and prays for guidance. Once he is gone, Brewster follows suit. The detective shadowing Halligan for his protection invites him home to supper, but they hear sirens. The Monastery of the Three Saints is on fire. Halligan runs back to rescue Brewster, who is fine. A beam falls, and he rescues Father Halligan. They clean up in the fountain of Trevi, then go to the deserted Coliseum, where Halligan asks why Brewster needs the whole day. Brewster wants to earn the total indulgence proclaimed by the pope for the 1950 Holy Year. He starts by making his first confession in 20 years to Halligan. The two then make their way to the sites, with Halligan -and the audience-learning more and more about Brewster. Halligan is still debating what to do when police see them. He helps Brewster evade arrest by ducking through an ancient door into a cloister where monks are working in the garden. In this religious order, the men remain enclosed for life and never talk to anyone except a superior. They are atoning for their sins and the sins of the world. Brewster finds himself drawn to the place during his brief visit: "Where I was, you could feel the hate in the air, but here..." As they leave, the abbot writes a note apologizing for everyone staring at them, but the iron latch that opened so easily for them has been corroded shut for 100 years. Halligan and Brewster head to the train station while the streets fill with police. Once there, they become separated, and the commissario finds Halligan. When he insists that Brewster is on a pilgrimage, the commissario goes to St. Peters, where they take him into custody and send him off in a van. Halligan, miserable at having inadvertently betrayed his friend, is convinced that the man is reformed: Then the news comes that Brewster has escaped, which leaves Halligan unsure of himself and his judgment. Retracing his steps, Halligan returns to the monastery. To his surprise, Brewster is there, wearing monk's robes. He asks the abbot: Did Brewster tell him his whole story? Is he a worthy penitent? The abbot nods. Brewster writes to Halligan: Prison was all past and no future, and this place is all future and no past. He asks when the next Holy Year will be. Halligan answers that it will be in 25 years, and he promises to visit then. A bell sounds, and they shake hands. Brewster steps back into the cloister and bolts the gate. Father Halligan strides down the hill to join the throngs walking toward the heart of the city.

On a voyage to Italy for the 1950 Holy Year celebrations, Pennsylvania priest John X. Halligan bunks with Joe Brewster, an affable conman who, unknown to John, has escaped from San Quentin. Joe, who was reared a Catholic, strikes up a friendship with John, but when the boat docks in Genoa, Joe steals his sleeping roommate's suitcase and disembarks in John's cassock to avoid waiting police. Joe's disguise fools them, as well as two priests who have come to meet John. The priests whisk Joe away to see the local sights, then place him aboard a bus filled with clerics bound for Rome. When John awakens, he must leave the boat in Joe's flashy clothes and is immediately arrested. At the police station, the commissioner assumes that John is Joe, until John recites the Preface to the Latin Mass, something only a priest could do. After being outfitted in new ecclesiastic clothing, John leaves for Rome with instructions to help the local police capture Joe. Meanwhile, Joe is befriended by Irishman Father McGinniss, who obtains a room for "Father John" in the Monastery of the Three Saints. Viewing the humble cells of the monks, Joe finds the quarters eerily reminiscent of San Quentin, but is content to have a hideout. When John arrives in Rome, he checks in with commissioner Aggiunto Bodulli, who speaks English peppered with Western idioms learned while a POW in Texas during the war. Bodulli wants John, who is the only person in Italy who can identify Joe, to help him, but is aware that John is reluctant to be a "stool pigeon." While the two men watch a procession, John is startled but says nothing when he sees Joe carrying a cross and helping a small, elderly priest carry his. Later, when Bodulli tells John that he will have a policeman pick him up the next day at the Monastery of the Three Saints, where he is registered, John realizes that Joe must be there. Unknown to John, Bodulli arranges for policeman Antonio Silesto to follow him. John arrives at the monastery as Joe and McGinniss are listening to a boys choir concert, and Joe is reminded of his days as a choirboy. When Joe is told that someone is waiting to see him, he thinks it is the police and is happy the visitor is John, to whom he apologizes. John wants to turn Joe in, but Joe convinces him to wait twenty-four hours. Later, as John walks through Rome, he realizes that Silesto is following him. Silesto apologizes for his ineptness, then invites John home to dinner to help him celebrate his wife's birthday. During their pleasant dinner, sirens are heard, and John learns that the Monastery of the Three Saints is on fire. He rushes to the monastery, leaving Silesto behind. John is relieved to find Joe, but is knocked out by a falling timber and is carried to safety by Joe. At the Trevi Fountain, a grateful John joins Joe for a carriage tour of Rome. At the Colosseum, Joe tells John that "the boys" were talking about a special pilgrimage to Rome's four major churches through which a penitent can have all of his sins forgiven. John tells Joe that forgiveness is only earned if the penitent is truly sorry, makes a confession and receives Holy Communion. Joe then asks John to hear his confession, and the two embark on the pilgrimage. The next morning, at the third church, St. Paul Outside the Walls, John gives a street urchin money for a relic he knows is phony, and Joe scolds him for putting the boy on the same path he was on as a child. While they are talking, Silesto, who has come to church with his wife, sees them and calls Bodulli. Although Joe and John have not seen Silesto, as they near Saint Peter's, they become worried when a pair of policemen stare at them, and run down an alley. Seeing a locked gate, Joe turns the handle and the pair find themselves in a monastery garden where an elderly monk uses a notepad to relate that they are all there for life, to pray and atone for the sins of others. Joe is puzzled by the air of love among these "lifers" and is shaken when the monk writes an apology for staring, saying that it is because the rusted side gate had not been opened in a hundred years. After Joe and John leave, Joe suggests that they split up so that John will not get in trouble, and the two take a taxi to the train station, where Joe slips away among a throng of priests. Bodulli and his men find John, and he unintentionally gives Joe's plans away by mentioning the names of the three churches they have just visited. Knowing that the fourth church on the pilgrimage is St. Peter's, Bodulli takes John with them to the Vatican. They arrive just after Joe has gone through the Holy Door, thus completing his pilgrimage. Joe does not resist his arrest, but soon escapes, disappointing John, who thinks that he must have been mistaken about Joe's true desire for repentance. Later, while the downhearted John is taking a walk, he again comes to the monastery gate and is surprised that it opens easily. Inside, he finds Joe tending the garden and angrily accuses him of using the kindly monks. Joe does not speak, but uses a notepad to tell John that he had been in a place with a past but no future and wants to spend the rest of his life in a place with a future but no past. The elderly monk then reassures John by writing that he knows everything about Joe and believes him to be truly penitent. Content, John gives Joe his most prized possession, his mother's rosary, and promises to work to get him a pardon in America and to visit him during the next Holy Year, in 25 years.