THEATRE / Testing the theory of relativity: Paul Taylor on the British premiere of Marvin's Room at the Hampstead Theatre, London

Here's a family that really puts the 'y' in 'dysfunctional'. Seen only as a dim form behind rippled glass tiles, the elderly father is gaga and bedridden after multiple misfortunes: stroke, cancer, blindness in one eye. 'That's what you get for not dying,' as Philip Larkin quipped about his own aged, invalid, mother. Then there's aunt Ruth (Carmel McSharry), a soap-addict in a peach trouser suit who was a martyr to pain before she had her brain rewired. The only snag is that when she fiddles with her electronic box, it activates the garage door so she feels the whole street knows her business.

Absentee-daughter and one- parent mother, Lee (excellent Phyllis Logan) might seem to have got off lightly. A cut-price glamourpuss with a false name and tight jeans that would look better on someone half her age, she is about to gain a degree in 'cosmetology' (special topic 'blusher' by the look of it). But because of some buried trauma, her relations with her older son Hank (Aidan Gillen) are cat- and-dog, to such an extent that he has burned their house down and landed in a mental institution, or 'loony-bin' or 'nut- house' as Lee prefers to call it, 'to show we have a sense of humour' about such things.

It's years since Lee last saw Bessie (Alison Steadman) her dowdy, unflaggingly good- natured sister who has had to cope by herself looking after this mini-Lourdes of a household. Then it's discovered that Bessie has leukaemia and that Lee and her boys may have the bone marrow needed for a transplant.

Of course, it's virtually an axiom in American film and drama that there's no family with problems so great that a little terminal illness can't cure them. Think of Love Story; think of Beaches. Marvin's Room by Scott McPherson, receiving its British premiere in an expertly judged and beautifully acted production by David Petrarca, is several cuts above such compulsory-kleenex jobs.

Part of this drama's approach to this painful subject, a family's tested loyalties in the face of death, has a sketch-like, loopy absence of piety and, yes, you laugh out loud at the play's wacko elements such as Bessie's aunt's crackpot efforts to prevent the father from noticing Bessie is no longer there. Since this involves pretending that the replacement nurse is a figment of his imagination, the poor man thinks he's lost his marbles.

At the same time, you brace yourself for those reassuring, feel-good features that are almost bound to crop up in an off- Broadway play about people who feel very bad indeed. The uncorrupted child in Bessie naturally gets through to the lost boy in her delinquent nephew (she even respects his right to be uncooperative about the bone marrow test). Meanwhile she and Lee begin to work on some of their unresolved hang-ups, in particular whether a life can be said to be wasted if entirely devoted to other people. When the transplant proves a no-go, Bessie suddenly realises that her life has been filled with love - not love received, but love it has been a privilege to give.

Preventing the sentimentality from seeming too on-cue and sickly are the fine performances, in particular the piercing, unaffected truthfulness of Alison Steadman's Bessie, her voice poignantly programmed to dispense selfless cheer, her prettiness smothered by ingrained frumpiness and the effects of illness. Even when the drama brings her ordeal into strenuous collision with kookiness (collapsing in Disney World, she is carried to the lost children's hut), she keeps a tight grip on the character's dignity. In a part that is almost the antithesis of her brilliantly manipulative Beverly in Abigail's Party, Steadman gives a performance that is equally devastating.