In the year 2000, Jorge Mario Bergoglio went to Germany to resolve
one of the most painful episodes of his life. The then Archbishop of Buenos Aires travelled there in search of an elderly fellow Jesuit, Franz Jalics.
When they finally met, for the first time in decades, an eyewitness said they fell into each other's arms and wept.
The two men had first crossed paths some 40 years earlier, when Jalics taught the young Bergoglio philosophy.
It wasn't long before the student became the older man's superior: just three months after taking his final vows Bergoglio was appointed head of the Jesuit order in Argentina.
Jalics and a fellow priest, Orlando Yorio, asked his permission to live among the poor in a city slum and he readily gave it. Two years later, in 1976, the military launched a coup and began what it called, with sinister euphemism, the Proceso de Reorganización Nacional.
Suspected Left-wing activists were captured, tortured, drugged and then pushed, still conscious, out of aeroplanes over the Atlantic.
One Sunday in May they came for Yorio and Jalics.
What happened next – and why – is the subject of an ongoing polemic that went global on 13th March this year, when Bergoglio was elected Pope.
Did he provoke the kidnapping by withdrawing his support for the priests after they refused his command to leave the slum?
Or did he work courageously behind the scenes to free them, broken but alive, five months later?
In Pope Francis: Untying the Knots, The Independent's Paul Vallely skilfully unravels the competing narrative threads, without ever oversimplifying either Argentine politics or the new Pontiff's complex personality.
It may surprise those who know Bergoglio only as the beaming, baby-kissing Bishop of Rome that he was once nicknamed the "man who never smiles".
Today, he deliberately avoids referring to himself as "the Pope", yet he was once seen as deeply authoritarian. He is famous for his low-key liturgies, but as a young priest he reportedly justified his High Church-style by saying: "Ordinary people like a touch of Evita."
He now enjoys incredible popularity, but other Jesuits disliked him so much that after stepping down as their leader he was sent to a lowly post 400 miles away from Buenos Aires.
He has said he wants a "poor Church for the poor", but he was suspicious of those, like Yorio and Jalics, who were inspired by Liberation Theology to live among the needy.
(When Bergoglio was made a bishop in 1992 Yorio left Argentina in disgust and died eight years later without ever being reconciled with the future Pope.)
It's not easy to explain all these apparent contradictions, but Vallely does so brilliantly. He shows how Pope Francis's personality was transformed in the crucible of the Dirty War, emerging with the old imperfections – aloofness, inflexibility and a taste for power – burned away.
When drug dealers threatened to kill one of his slum priests four years ago, he acted without hesitation, telling the cleric: "If someone has to die, I would prefer it be me" and offering to sleep at his house.
Almost every page of the book contains this kind of telling detail, which Valley has gleaned from pounding the streets of Buenos Aires.
Not surprisingly, there are a few signs of hasty publication: there is some repetition and the final chapter on the papal reform programme is inevitably quite sketchy.
But for such a sophisticated biography to appear now, less than six months after the papal election, is little short of a publishing miracle.