Observations: Trilogy is the feminist Fringe hit that puts the treat into treatise

Amid all the braying comedians and C-listers slumming it on last year's Edinburgh Festival Fringe, the show that really got people talking was a thoughtful piece of performance art by a young, Glasgow-based feminist. Nic Green's three-hour-long Trilogy, which arrives in London this month, is that unfashionable thing, a feminist treatise. On paper, it shouldn't have been a hit, but audiences loved it. Bloggers and reviewers hurled superlatives. The Poet Laureate came to see it, and brought her 14-year-old daughter with her.

Trilogy looks at the past, future and present of feminism. It engages with the energy of the first-wave 1970s movement, using footage of the 1971 debate filmed as Town Bloody Hall. However, it also asks simple questions about the state of contemporary female self image.

At one point, 50 naked women, all local volunteers, march on stage in a thrillingly life-affirming dance. It's not the nudity that shocks but the realisation that we just don't see female bodies like this, anywhere in the media. They've been airbrushed out of sight, and we don't question it. But this is what female bodies look like: they droop and point and age and bulge; they ripple with sinews and capability. And marching, stomping and punching the air in time to Pixies' "Broken Face", they're beautiful.

"There were women in the group [of volunteers] who said things like, 'I've never even been naked in front of my boyfriend'," says Sophie Younger, a mother of four sons between 15 and 21, and one of the oldest volunteers to take part. "It drew people who wanted to do it as a cathartic thing for themselves because they'd always had hang-ups about their body, as well as women who had been drawn in by the profundity of what Nic Green was trying to achieve."

If parts of the audience perhaps turned up because they'd heard the production contained extensive female nudity, they stayed because they were being charmed, entertained and challenged. The passion and charismatic conviction the performers bring to the piece really has to be experienced first-hand.

"I had no sense of vulnerability, or of being a woman dancing naked for an audience, but neither did I feel 'empowered'. I just felt an absolute joy to be dancing to the Pixies, on that stage," says Younger. "It only wavered if I met audience members during the interval: I'd think, well, I haven't seen you naked, so you've got a slight advantage there! However, I am very happy to think that Carol Ann Duffy has seen me naked. That thought has carried me through the year."

Trilogy at BAC, London, 13 to 16 January; Barbican, London, 22 & 23 January; Nuffield Theatre, Lancaster, 30 January (Nicgreen.org.uk)