Walking on Water, White Bear Theatre Club, London

A couple of years ago, eagle-eyed visitors to the Edinburgh Fringe were surprised to discover a doyenne of British film and stage, Susannah York, hiding away in a production of The Deluge at a minor venue of the city. Now, she's doing it again, this time appearing in the back room of the White Bear pub in Kennington, south London.

This bijou theatre, which holds no more than 45 people, on seats apparently recycled from the Barbican's Pit, is celebrating its 20th anniversary this year. York, as elegant as ever with her devastating cheekbones, may be the main reason that people will venture to this fringe venue, but those intrepid enough to wander off the beaten track will find themselves rewarded with a rather good evening's theatre.

York plays Mama Palmer, whose once sharp mind is rapidly being enveloped in the fog of Alzheimer's disease. We first meet her wielding a muddy spade as she returns triumphantly from the garden where she has been digging up the remains of her dead dog. Before this, she has escaped from her home in the middle of the night to uproot gravestones in the local cemetery and set herself on fire. Her older daughter, Frances, has been caring for her single-handed but, having reached crisis point, she summons her younger sister Betsy back from Los Angeles to the old Indiana family home to help out.

At first it's a battle of wills between the frivolous Betsy (the hilarious, brittle Mel Hudson), who denies her mother's dementia, and her dowdy, dipsomaniac spinster of a sibling Frances, who is hopelessly resigned to it. The play really takes off in the second half, when Betsy's teenage daughter Henny (the excellent Victoria Yeates) enters the fray.

Paul Minx's four-hander deals tenderly with the trials of Alzheimer's, but goes deeper into a moving examination of the mother-daughter contract and the toxic potential of secrets.

Although some 20 minutes too long, and with some hackneyed plot twists, Minx's writing appealed to me very much. He finds a convincing voice for each one of these women, across the generations, from the confused aggression of Mama to Betsy's Cali-nonsense ("I'm future-oriented," she squawks when her sister accuses her of falsely building up her achievements). But best of all is Henny's comical teen-speak – a bad case of dictionary-itis combined with slang and wisecracks, straight out of Dawson's Creek.

Thanks to some clever casting by Lolly Susi, and uniformly good acting, the four actresses add up to a highly believable family unit. It's a production worthy of a larger venue, such as the Bush – and a very good start to the White Bear's 20th- birthday celebrations.

To 28 January (020-7793 9193)