Based on the diary Pope John XXIII kept between the ages of 14 and 18, his lifelong concern for tolerance, the underprivileged, and world peace is told. Rod Steiger, in the central role, acts as "intermediary" between the Pope and the audience, interpreting John's words, thoughts and actions. Steiger visits the actual places in which John lived, recreating the conditions, environment, and forces that affected his development. Through this unusual technique, we are brought close to an extraordinary man - one who's able to win the love of many people of all faiths.
| Actor | Role |
|---|---|
 Rod SteigerThe Intermediary | The Intermediary |
 Adolfo CeliMsgr. Radini Tedeschi | Msgr. Radini Tedeschi |
| GGiorgo FortunatoSegretario del Nunzio Roncalli | Segretario del Nunzio Roncalli |
| OOttone CandianiIl Prete Peccatore di Venezia | Il Prete Peccatore di Venezia |
| AAlfonso OrlandoIl Parrroco | Il Parrroco |
 Alberto RossiAngelo Roncalli - Age 7 | Angelo Roncalli - Age 7 |
| GGiovanni RossiAngelo Roncalli - Age 10 | Angelo Roncalli - Age 10 |
| FFabrisio RossiAngelo Roncalli - Age 4 | Angelo Roncalli - Age 4 |
 Pietro GermiPontiff's Father | Pontiff's Father |
| RRita BertocchiPontiff's Mother | Pontiff's Mother |
| AAntonio BertocchiUncle Xavier | Uncle Xavier |
| AAntonio RuttigniDon Pietro, Parrocco di Carviso | Don Pietro, Parrocco di Carviso |
 John XXIII.Self (archive footage) (uncredited) | Self (archive footage) (uncredited) |