Theatre The Thickness of Skin Royal Court Upstairs, London

You would have to be very thick-skinned indeed not to wince with recognition during Clare McIntyre's The Thickness of Skin. Premiered now in Hettie Macdonald's beautifully acted production, it's this author's first play in six years and it quickly establishes itself as the most searching look at the problems, perils and mixed motives attendant on offering help to the unfortunate since David Hare's The Secret Rapture (1988).

Well-heeled Michael (Rupert Frazer) thinks that simply because he pays income tax he's entitled to an exemption from considering the needs of people whose lives are in a mess. His like-minded wife Roanna (Elizabeth Garvie) is duly appalled when she discovers that their 17-year-old son (Toby Ross-Bryant) has been doing good turns for the unkempt, scene-creating schizophrenic woman (Maggie McCarthy) who has a bedsit in the house next door. "Never, never get involved with somebody like that," Roanna snaps. "You don't know where it's going to end."

It is Michael's schoolteacher sister, Laura, whose painful experience might seem to bear out this ruling, although the play implies that things needn't have been so bad if she could have acknowledged from the start that pure altruism is often an illusion and that, in the relationship between benefactor and recipient, it can be extremely tricky deciding who is using whom.

Quite against the rules of the day centre for homeless people where she does voluntary work, Laura has taken in one of its clients. It's no great obstacle to this act of charity that Eddie (Mark Strong), an out-of-work northern carpenter, happens to be a sexy bit of rough, or that Laura is a lonely, thirtysomething divorcee. But the play does not invite an easy cynicism. Her earnest, well-scrubbed sincerity and middle-class wholesomeness shining out in Amelia Bullmore's splendid performance, Laura is, you feel, a genuinely good woman with an anguished social conscience.

"I don't know what I want. What does what I want matter? We're all brought up on bloody want... What do you want for your birthday? What do you want for your wedding?... Do you want to forget your birthday? Do you want to forget thinking so hard and constantly about what you want?" Insofar as it's an attack on greed, Laura's long comic rant in the second scene does her credit, but insofar as it's symptomatic of a woman who needs to think that her own needs need not come into play, it augurs ill for her fundamentally unequal relationship with Eddie, who has lost everything including access to his little daughter.

An ironic fate, then, for such a heroine, that through a series of misapprehensions, she should end up being ritually humiliated by her drunken ex-lover in front of an appalled family, her attempted kindness dismissed as "all middle-class wank. Do something for some poor sod like me. Feel good about yourself... and get a fuck into the bargain." It's an excruciating moment, as is the scene where the schizophrenic woman, hideously dolled up, seeks out the 17-year-old at his public school in order to give him a shop cake. Which of us, in such circumstances, would have the courage to keep faith with the weak and helpless? But is that any argument for not getting involved in the first place?

n To 20 April. Booking: 0171-730 1745