The film stars Anthony Quinn, Oskar Werner, David Janssen, Vittorio De Sica, Leo McKern, John Gielgud and Laurence Olivier.

Composer Alex North won a Golden Globe for his score, and the film was nominated for Best Motion Picture Drama.

During the height of the Cold War, Kiril Pavlovich Lakota, the Metropolitan Archbishop of Lviv, Ukraine, is unexpectedly set free after 20 years in a Siberian labour camp by his former gaoler Piotr Ilyich Kamenev, now the Premier of the Soviet Union.

When the pontiff suddenly collapses and dies, the process of a papal conclave begins, and Cardinal Lakota participates as one of the electors.

Meanwhile, the world is on the brink of nuclear war due to a Chinese–Soviet feud made worse by a famine caused by wide spread crop failures in China.

The evening after his election, Pope Kiril, with the help of his valet Gelasio, sneaks out of the Vatican and explores the city of Rome dressed as a simple priest.

A major secondary plot in the film is the Pope's relationship with controversial theologian, philosopher and scientist Father Telemond (who resembles real-life priest Teilhard de Chardin).

To the Pope's deep grief, Father Telemond dies from a neurological malady, shortly after giving the former some much needed support.

Later, the Pope returns to the Soviet Union, dressed in civilian clothing, to meet privately with Kamenev and Chinese Chairman Peng to discuss the ongoing crisis.

This made him reluctant to sell "anything other than a thriller or a very simple story to the movies again" because of the way Hollywood "tends to distort the underlying philosophy and theology of anything that can't easily be shaped for the screen.