Untouched, King's Head, London

As a theme for drama, asexuality – that's to say, lack of interest in, or desire for, sex – surely languishes near the bottom of the heap of sex-related topics. This is probably because it's an issue that's defined by negatives. Asexuality relates to frigidity as indifference does to apathy. The frigid and the apathetic may well mind about their condition and acknowledge that they might benefit from help.

There are two areas, though, where asexuality offers a playwright purchase and potential. One is the issue of rights. Just because it's hard for sexualised beings to grasp, why should asexuality be stigmatised as suspect or perverse? The other relates to questions of psychology and identity. Some might argue that, unless thrown into a position where their beliefs about themselves are tested, how can asexual people make an informed choice about their defining preferences?

It's to the credit of Untouched, a 40-minute drama by Nick Huntington, that it makes you think about both of these issues. But it's a shame that it does so by the wrong kind of provocation. I have no doubt that this is an honourable, well-intentioned piece of work. Dramaturgical clumsiness, not bad faith, gives the piece its slightly exploitative air.

The setting is a studio in a converted attic. To this, her workplace, Miriam (Miriam Lucia), a young photographer, brings Caleb (Ed Fromson), a hunk with a great deal of photogenic sexual preference. She's just picked him up at a local pub and has offered to pay him to pose, as she wants to capture the real deal as opposed to the standard model-type.

Caleb is clearly a mental cut above the building site where he works. Stripping to the waist, he lets Miriam reposition his limbs and send her lens on intimate forays around his person. He gets ideas; he thinks she has ideas. And her mother (Tracey-Anne Liles) gets ideas, too. She finds out that Caleb was once charged with date rape. She threatens to blow his gaff if, for £5,000, he doesn't... well, you get the picture. The mother is convinced that Miriam is frigid rather than asexual and that the history of this began when, as a child, she discovered the corpse of her killed brother.

Jonathan Guy Lewis directs a dignified, well-acted production but the set-up pushes the piece into the chiaroscuro of melodrama rather than into the grey areas of genuine ambiguity. The pay-off, though the right one, is too abrupt to pay dividends. That said, Untouched does linger in the mind.

To 2 December (020-7226 1916)