Professional actors from Pimlico Opera worked alongside inmates to stage 16 public performances of the musical Guys and Dolls. At the same time, BBC Wales has been filming the three-month process, from early rehearsals through to opening night, and BBC 2 will broadcast the result, Guys, Dolls and D-Wing, at 8pm tonight.
Richard Trayler-Smith, director of the documentary, says his team shot 50 hours of film to make the programme. 'It was certainly one of the most moving experiences that I have had as a film-maker,' he says, 'to see the way in which those guys responded to being in (the show), to see how their confidence grew, their self-esteem grew; to see, to put it very simply, the light dawning for them.'
The documentary producer Hilary Boulding came up with the idea of making the programme after seeing Pimlico Opera do West Side Story at Wandsworth last year. It was the irony of seeing a very lively, professionally presented show in such a grim setting that generated the idea.
Having completed the film, Ms Boulding is convinced that it is more than just a television programme or the story of a production.
'It is,' she says, 'a very powerful human story, seeing these guys actually start to reassess themselves, as they do in the (film).'
Pete McGing, a prison officer, who played alongside the inmates in Guys and Dolls (and who was, incidentally, the only guard who volunteered to do so) is also convinced that the experience has indeed made an impact on the prisoners who took part.
'I've seen some really excellent attitudes among the men,' he says. 'They've grown up, a lot of them.'
David, a former share dealer serving time for insider trading, worked backstage on the production. He believes the show has had a profound effect largely because the prisoners were being given the opportunity - most of them for perhaps the first time in their lives - to do something that was really worthwhile.
Most of the prisoners had never sung or danced on stage before. 'I thought it would be like a school play or something silly in the church,' said Shaner Osman, 21, serving eight years for robbery. Few, if any, had even set foot inside a theatre.
'It is, quite simply, alien to their culture,' explains the conductor Wasfi Kani, Pimlico Opera chief and the driving force behind the project. It was not, in fact, until the week running up to opening night that it actually dawned on the men that they were going to be decked out in smart costumes, with a bank of 80 lights on them, while being put through - to the accompaniment of a 15-piece orchestra - their impressively intricate paces.
Repeatedly I heard from the inmates that the feelings they derived from actually learning and remembering what they had to do on stage gave them the kind of buzz no drug or well-executed crime had ever provided.
'The play has really opened up a new kind of world and new thoughts for me,' says Che Cullel, a 21-year-old convicted burglar who admits his extensive criminal career had become increasingly violent.
'I've definitely got ideas and motivation from being in it. For starters, it's given me determination, and it's given me patience, because before I had neither. It's just shown me talents and qualities I didn't even know I had. I can go out there and I can make people laugh. From that, I know now that I can go into further things. And I will do.'
And following a visit to Wandsworth by representatives of a professional stage company to see Guys and Dolls, two of the inmates have been asked to go for an audition on their release.