The Maths Tutor, Hampstead Theatre, London

A play called The Maths Tutor sounds about as enticing as a piece entitled The Traffic Warden or The Emergency Dentist. But don't be put off by the name, for the deeply painful yet obstinately funny situation at the heart of Clare McIntyre's latest drama throws an intriguing light on the sexual prejudices and practices of our day and on their consequences for family life.

The scenario has distinct echoes of The Children's Hour, the 1934 play by Lillian Hellman in which a neurotic and malignant young girl at a New England boarding school runs away and justifies her action to her family by claiming that the schoolmistress is having a lesbian affair. It's a lie that unwittingly stumbles on a truth - forcing to the surface, in the accused couple, agonising doubts about sexual preference.

In McIntyre's piece, a similar false allegation is made by a 15-year-old boy, JJ (Ben McKay), who, with his friend Tom (Nicholas Figgis), is having weekly extra maths lessons. Lazy, fed up, and angry with his mother for having kept something important secret, he tells her that the tutor has been touching him up and masturbating. It's an outrageous fabrication and is successfully refuted, but not before it has exposed the fact that the tutor, Brian (Martin Wenner), is having a highly sexual affair with Tom's father, Paul (Christopher Ravenscroft), and that an apparently contented marriage is built on a fault-line.

McIntyre cunningly compares and contrasts two different family set-ups. Anna, JJ's glamorous estate-agent mother, has brought him up on his own since her divorce. Wittily and warmly played by Sally Dexter, she's a woman who likes a glass or six, is never without a lover, says the first thing that comes into her head and prides herself on being upfront about sex. She fails to realise that her determined frankness has probably pressurised poor JJ into bragging, quite without foundation, that he has lost his virginity. The limits of this swinging freethinker's liberalism are amusingly tested by developments. Draining a vodka bottle, she rambles on about how paedophilia is everywhere these days: "Well, maybe not the RAF. But definitely the Marines. Royalty. And as for the telly..." When the tutor demonstrates his innocence and scotches the idea that one can equate homosexuality with paedophilia, her idea of a friendly, broad-minded parting shot is to say: "If it's any consolation, Brian, I didn't even realise you were gay."

A troubled single parent, Anna thinks she envies the child-centred placidity of the domestic set-up shared by Paul and his wife, Jane, who is portrayed with great empathy by Tricia Kelly. The children, though, are the only thing that keeps their marriage going. Paul often leaves for adulterous weekends, and Jane is prepared to tolerate the situation, provided she never has to come into contact with any male lovers. In introducing Brian to the home as a tutor, her husband has broken the golden rule, and the dam of her stoical acceptance bursts. Gently and without any inflated claims that they are going to be sisters in solidarity, the play charts a growing rapport between Anna and Jane, women so different that they barely like each other at the start. It's typical of Anna's tactlessness that, at one point, she mentions Jane's fortysomething facial hair. Tellingly, it turns out later that Paul hadn't even noticed. And when, at the end, she goes off to have it removed by electrolysis, she is, at long last, doing something to please herself.

Anthony Clark directs a fluent and expertly inflected production that augurs well for his new regime at Hampstead Theatre.

To 25 October (020-7722 9301)