Three years after its sensational debut at the Edinburgh Festival, David Harrower's controversial play Blackbird is out on a national tour which this week brings it to the Oxford Playhouse. Considering the plaudits it gathered during its West End run, I was surprised to find the theatre less than bursting on the first night, but there can be no denying with what rapt attention the two-thirds-full house greeted this gripping play.

Blackbird, I regret to say, is one of those pieces designed to be performed without an interval. With a running time of 1hr 40min, considerable demands are made on the bottoms and bladders of the audience, and even more so on the staying power of the two actors who carry the drama almost without assistance. Expertly directed by David Grindley, Robert Daws and Dawn Steele rise to the challenge brilliantly, delivering performances which, while painful to watch at times, remain utterly compelling.

They play two former lovers reunited after 15 years in the rubbish-strewn canteen of Ray's place of work, whither Una has tracked him down after seeing a picture of his 'team' in a trade magazine. The mess, presumably, is a metaphor for their messed-up lives. There is a difference from the usual lost love scenario, you see. When Ray affects to believe that Una might not be who she says she is, she snaps: "How many other 12-year-olds have you had sex with?"

Aged 40, we learn, Ray had fallen for the temptations offered by a neighbour's daughter, been caught for it, sentenced to three very painful years inside, after which he set about rebuilding his life under a new name in a new job (the nature of which remains an intriguing puzzle until almost the play's end).

To what extent the temptation was actually offered is the sensitive issue at the centre of the drama. The girl's complicity in the crime - now admitted, now denied - is the uncomfortable aspect of the play that must worry all who see it. Una, we learn, did her best to reduce her partner's culpability when the case came to court. "I'd have got ten years, if they'd known that," says Ray after being reminded of the premeditated, carefully planned and long-practised nature of their intimacy. In airing the disturbing possibility that love can exist in these circumstances, the play goes to the heart of the debate over paedophilia.

Its verdict - as it must be - is that it is always wrong, but a number of profound questions are raised that will be debated by members of the audience long after the curtain has fallen.

Blackbird continues until tomorrow (01865 305305, www.oxfordplayhouse.com).