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At the start of 2005, David Harrower told me about the play he was writing for the prestigious Edinburgh International Festival. He had just gone through an about-turn and ditched his first draft with its roll-call of 18 characters. Throwing it in the bin was difficult, he said at the time, but he had started again, trying to write a lean two-hander that would focus entirely on the central dilemma: the reunion of an older man and a younger woman 15 years after the under-age affair that put the man in prison.

"I thought for the Edinburgh International Festival I should have a cast of no less than 30, but mid-December the whole thing shattered and I was left with two characters," recalls the Scottish playwright. "I realised I was building this huge structure but there was only one thematic kernel that I needed which was between these two people."

At that point, it was a full 10 years since Harrower had become Scotland's biggest theatrical export with his debut play, Knives in Hens, translated into more than a dozen languages, staged in 25 countries and given around 80 professional productions worldwide. I idly speculated whether his newspaper name would ever change from "David (Knives in Hens) Harrower".

"That day will come," he promised.

A matter of months later, he was proved right. In August 2005, Blackbird opened in Edinburgh with Roger Allam and Jodhi May as Ray and Una, trying to come to terms with the sexual affair they'd had when Ray was a grown man and Una was just 12.

Peter Stein's production was an immediate hit. The Scotsman called it "mighty and timeless", the Guardian said it was "a riveting study in sexual obsession" and the Daily Telegraph welcomed it as "the most powerful dramatic two-hander since David Mamet's Oleanna". When the production transferred to London, the reviews were even more ecstatic, leading to an Olivier Award for best new play in February 2007, pipping to the post Tom Stoppard's Rock 'n' Roll and Peter Morgan's Frost/Nixon.

A month later, it opened off-Broadway in a production starring Jeff Daniels. The New York Times called it "a drama that promises to be the most powerful of the season".

By this time Blackbird had been seen in Berlin, Vienna and Helsinki with productions lined up everywhere from Chile to Japan. Now Australian star-turned-director Cate Blanchett has staged the two-hander to similar acclaim for the Sydney Theatre Company where she recently became joint artistic director with her husband, Andrew Upton.

"Blackbird is shocking, compelling and morally challenging," said the Australian newspaper, praising Blanchett's "great sensitivity for the rhythms and silences of Harrower's spare, fragmented but theatrically poetic dialogue".

All of which means her production, starring Paula Arundell and Peter Kowitz, will be one of the most talked about shows in Wellington when it arrives at the New Zealand International Arts Festival next month. The debate will continue in September with a new staging by the Auckland Theatre Company directed by Margaret-Mary Hollins and starring Michael Hurst.

It's safe to say the playwright has been reborn as "David (Blackbird) Harrower". "Are you pleased about the success?" I ask him, when we meet for a drink around the corner from his Glasgow flat.

"What do you think?" smirks the shaven-headed playwright, a soft-spoken 40-year-old with a quiet sense of humour. He is not given to hyperbole, but his delight is obvious.

"I look at Blackbird now and it's like a fruitcake," he says. "It's full of stuff, however stripped back you think it is. I've had lots of letters, only from women, saying they've seen many abuse dramas over the years and nothing has got close to the complexity of feelings that they felt in their similar predicament when they were children having a relationship with an adult. I'm not saying they validate what I've written, but it's nice to know you've done something."

What Harrower does in this electrifying play is sidestep the familiar Sunday supplement debates about the evils of paedophilia and open up alarmingly grey areas. He does this without condoning an illegal relationship, yet keeps us on the edge of our seats as we shift allegiance from Ray to Una and back again as the complex layers of their history are unpacked.

Going into places you wish it didn't, it is an uncomfortable drama that encapsulates the unresolved, misshapen, emotional mess that might accompany any suddenly interrupted relationship, legal or otherwise. Blackbird is a disturbing work because it shows us the muddiness of water that we like to think is clear.

Harrower accounts for the success of Blackbird in terms of the mystery at its heart. "Some of my other plays were a bit more prosaic, a bit more tempted to see the truth rather than stay away from it. You can get plays that go to the centre of things and tell us what the centre looks like, but the plays I enjoy - Beckett, Shakespeare, Bernard-Marie Koltes - have mysteries. I've realised my own voice is a bit more hushed and avoiding than I thought it was."

He is relishing the continued international acclaim, although it does have its downside. "My work schedule has quadrupled," he says, just before racing home to meet his next deadline. "And I've still got plays to write for theatres that commissioned me seven years ago."