In a downtown restaurant in Milan, a group of actors are celebrating the first performance of their new show.

As one might expect for a musical version of Herman Hesse's Buddhist novel, Siddhartha, the cast of what is an unashamedly commercial mix of Bollywood and pop video theatrics are young, beautiful and bursting with post-show energy.

Earlier that evening, the young stars gave a dynamic performance of Siddhartha - The Musical at a huge theatre complex in front of an invited audience of friends, family and assorted co-producers of the show, including representatives of the New York-based Broadway Asia International. Such serious interest in the play bodes well for Siddhartha - The Musical's Edinburgh showcase, which opens at the end of the month as part of the Assembly Rooms Edinburgh Festival Fringe programme, putting an international spotlight on something which has already wowed audiences in Italy and beyond.

Overseeing the post-show festivities with equal ebullience are writer/director Isabella Biffi and producer Gloria Grace Alanis. Biffi is an Italian musical star, Alanis a Mexican former model who settled in Italy. With successful careers under their belts, somewhere along the way the two women bonded over Buddhism. Together they are a force of nature, and take Siddhartha - The Musical very seriously indeed.

"It is the message of the performance that is important," Alanis translates for Biffi in excitable, broken English. "Whatever happens to you in the world, love and peace are the important things."

These might sound like lofty ambitions for a commercial musical, but, with Biffi nodding in agreement across the dinner table as the pair flash the widest of smiles, you get the sense that there really is something deeper at play here.

The next morning in Milan's Opera Prison is as far away from the glamour of the night before as one can imagine. It is here, however, among the 1400 inmates of one of the biggest maximum security prisons in Europe, that Siddhartha - The Musical began. Ushered into a large, if somewhat more makeshift theatre space than the lavish arena the play was performed in the night before, we're greeted by seven inmates, who perform several scenes from Siddhartha for us.

What we see may be rougher in terms of physical technique, but in terms of spirit, determination and rough-hewn athleticism, the Opera performance adds a new resonance and depth to a show which is effectively about one man's quest for self-knowledge as he goes about the world. In this respect, this version of Siddhartha - The Musical is theatre at its purest.

When the performers sit lined up across the stage to talk to us, it's hard to equate these focused, beatific and near evangelical-sounding men with the crimes they have committed. Given that 1300 of the 1400 prisoners confined in Opera are serving life sentences, those crimes must be very serious indeed.

Yet, when the men talk, while there's a certain understandable swagger to their bearing which isn't that far removed from the professional actors letting off steam the night before, they sound transformed, as well they might.

Siddhartha - The Musical was developed by Biffi with the prisoners as part of an ongoing theatre programme that has presented a series of shows over the last seven years.

With many of the men being involved in the project from the start, you get the sense that Siddhartha has been the pinnacle of their achievements thus far.

Many of these former hardened criminals have clearly softened over the years, and some have themselves become Buddhists.

It was the prisoners who suggested to Biffi and Alanis that they take the show out into the world in a way that they will never be able to perform it.

"Before we did these workshops," says one man who we've just watched play the Narrator as if his life depended on it, "a lot of therapists came in, but we are lifers for a reason, and they couldn't get through. But the workshops opened my heart."

In the clamour to talk to a rare audience not made up of fellow prisoners, the same message, again translated by Alanis, comes over again and again.

"Before," says a younger inmate, "I was a bad boy, but when I joined the workshops I became another person. I'm still a boy," he laughs, "but I'm a good boy now."

This is the message Biffi and Alanis were so keen to explain the night before. As one of the prison performers puts it: "We want to give the message to the world that everyone can have a second opportunity. When I get out of prison, I don't want to be seen as a prisoner anymore. I want to become a good citizen, and do something of value."

The nearest comparison with such a set-up is with Barlinnie, the Glasgow prison that set up a radical art-based rehabilitation programme in the 1970s.

That scheme transformed convicted murderer Jimmy Boyle into an artist and became ­something of a cause celebre before being quietly wound down.

In Milan, however, under Biffi and Alanis's guidance, the message of Siddhartha - The Musical looks set to go on.

Future plans include touring the show to 28 prisons in Europe. A docudrama is also being planned to tell the story of how taking part in the prison theatre project changed the men's lives, as a group.

How any of this will translate to audiences watching the professional staging of Siddhartha - The Musical in the hurly-burly of Edinburgh in August remains to be seen. Whatever happens, it has arguably already achieved its goal.

"We are so grateful to have been part of this community," says the inmate who plays the Narrator in the Opera prison version.

"We're aware that we've done bad things in the past, but now we are a part of this, we can enjoy a new life."

Siddhartha - The Musical, Assembly Rooms, July 31 to August 24, www.siddharthathe