Picture the scene: a fleet of black limousines crunches up the driveway of a Buddhist temple nestled in lush pine-carpeted mountains an hour west of Tokyo. The precious cargo of limousine one – a violent but ageing mob boss – steps out into the sun, surrounded by four sumo-sized bodyguards and is welcomed by a priest. As cherry blossom petals blow gently in the wind, the gangster enters the shrine and proceeds to be solemnly ordained into the Buddhist priesthood.

It sounds like the opening of a terrible yakuza movie, but this is what took place in this picture-perfect setting when Tadamasa Goto, one of Japan's most feared mob bosses, stepped out of the shadows this week and into the path of God.

Unsurprisingly, he was watched – at a safe distance – by a 40-strong media scrum. It was as if the infamous mafia don John Gotti, a man with whom Goto is sometimes compared, had ditched his dapper suits for priests' robes at the local Catholic church.

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Today, Goto reportedly spends his time praying and contemplating on long meditative trips into these mountains. Eventually, he could find himself comforting the sick and the bereaved, an odd occupation for a man who dealt in murder and mayhem for Japan's largest yakuza syndicate, the Yamaguchi-gumi.

"I believe he has really turned away from his past life and wants to change," says Jishu Tsukagoshi, one of the young priests at this temple, the Joganji. "But only time will really tell if that is true."

Probably nobody except Goto himself knows why he decided to swop a pistol for an incense burner, though there is no shortage of theories. Most observers scoff, however, at the idea that the 66-year-old, known as one of the country's most dangerous gangsters, had a genuine change of heart. The most popular theory is that he is on the run, hoping to escape assassination by former yakuza colleagues infuriated that he brought them one of the few things they genuinely fear: publicity.

Goto was splashed across the world's newspapers last year after details emerged of a deal he had struck with the US authorities. In exchange for a queue-jumping transplant at the UCLA Medical Centre in Los Angeles, Goto reportedly agreed to become an FBI informer.

For the bureau, he was a rare prize: a high-ranking boss apparently willing to break the yakuza's silence about its operations in America, and its drug and weapons connections with North Korea.

But Goto walked away from the deal with a new liver having reportedly given the FBI nothing that they could not read in a 400-yen (£2.70) Tokyo magazine. "He came to the States and got a liver and was laughing back to where he came from," the former FBI chief Jim Stern told the LA Times. "It defies logic."

That sleight of hand may have saved Goto from being saddled with the deadly tag of snitch, but not from mob concerns that he couldn't keep his name out of the press.

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Those fears were compounded last year when he invited five well-known popular enka singers to his birthday party outside Tokyo on 17 September, igniting a media scandal when it was aired in a popular weekly magazine. The leadership of the 40,000-strong Yamaguchi-gumi reacted furiously, officially banishing him from the group during a meeting at its headquarters in Kobe and declaring him persona non grata.

According to mob watchers, Goto took with him a faction of 1,200 men, a reputation for violence and a drastically reduced life expectancy.

"There are a lot of people that want to kill him these days," says Jake Adelstein, the Japan-based journalist who broke the story of Goto's liver transplant.

"There's even kind of a bidding war between groups, and not just the Yamaguchi-gumi, because the bounty on his head is quite substantial."

Observers believe that Goto took trips abroad and played a lot of golf before hew decided that his new life was, in the words of the weekly magazine Friday, "empty and lacking stimulation."

The prospect of a gang war between his breakaway group and the Yamaguchi-gumi also caused him sleepless nights.

His ordination gives him the chance to appear remorseful, and hopefully stave off the chances of being whacked.

For the 400-year-old temple, however, the attractions of the arrangement are less clear. Its head priest, who has a two-decade relationship with the don, has taken a pounding in the media, which finds Goto's sudden conversion to Tendai Buddhism a little hard to swallow.

How is a man who has wallowed in the demi-monde for most of his life supposed to achieve Buddha-hood this late in the piece?

Perhaps cash has helped smooth the way. Goto donated $100,000 (£68,000) to the LA hospital that performed his transplant, earning himself a commemorative plaque that reads: "In grateful recognition of the Goto Research Fund established through the generosity of Mr. Tadamasa Goto."

The UCLA Medical Centre came under intense fire when it was revealed that Goto and three other yakuza had jumped over several hundred waiting patients to get their new livers. Those patients all died.

Joganji's head priest is keeping mum about rumours of financial compensation. But he admitted in a magazine interview this week that the don's epiphany was slow in coming. "At first, he appeared to have little interest in Buddhism but as he got older, it deepened."

Goto's life of crime presumably left little time for the pursuit of enlightenment. Apart from drugs, fraud and prostitution, he has been linked most infamously to a vicious knife attack on Juzo Itami, a top director who angered the mob by portraying them as lowlife thugs in a hugely popular movie. Itami subsequently committed suicide in a death that has long been rumoured to have been a mob-hit in disguise.

In 2006, Goto and his oldest son Masato were arrested for real-estate fraud following the killing of a man who was trying to clear out Goto-gumi gangsters. He was acquitted but faces other charges.

Those court battles, his declining health and the prospect of war with probably the world's largest organised crime outfit may all have played a part in his conversion, believes the temple priest Koji Tsukagoshi. Whatever the reason, he says Goto is a shadow of his former self. "He looks just like any man of that age, not scary at all, until you see the men he keeps around him."

Goto offered few clues at his ordination, reportedly brushing off questions about his decision with a curt statement: "Buddha will make me his disciple and enable me to start a new life." Adelstein, who went into hiding in fear for his life after his story was published, recommends that his tormentor fill out an organ donor card and give his liver to a "more deserving" person.

"It would be a good and final chance for him to achieve some absolution. Because I think that the odds are he's going to be experiencing reincarnation first hand, a lot sooner than he expects."

In the gang: The Yakuza

*The Yakuza, a loose alliance of Japanese criminal gangs, trace their origins back to a thoroughly romantic source: the machi-yokko, mercenaries who protected townsfolk from bandits in the 17th century. Others connect them to the kabuki-mono, masterless samurai who terrorised villages across the country.

*The violent gangs to whom the modern term refers emerged after the Second World War, when they asserted control of the black market. They conduct themselves with a veneer of honour and politesse, but the racket is a familiar one: smuggling, selling drugs, extortion, illegal gambling and prostitution rings.

*The Yakuza's organisation follows a highly sentimentalised father-son structure, where relationships are cemented by the mutual drinking of sake in a ritual known as Sakazuki.

*In the gangs' mythology, the archetypal Yakuza is an abandoned son taken in by a father figure; gang members cut all family ties to affirm their allegiance to their superiors.

*Dedicated Yakuza confirm their loyalties with ornate tattoos (left); if they err, they expect to lose their pinkies. With about $13bn (£9bn) flowing through their coffers each year, it is little wonder that 40 to 80,000 people are prepared to take the risk.