Bea, Soho Theatre, London

After plays on ego, emotion and religion, the writer-director Mick Gordon has turned the attention of his On Theatre project to assisted killing; but not, as you might expect, to consider the case of a middle-aged or disabled sufferer.

Teenaged Bea is full of bounce and brio, even though she has been more or less confined to bed for eight years, thanks to an unspecified illness. What we are seeing over a dense 95 minutes is the physical expression of an inner life, a spiritual fantasy.

This seems to be an excellent way of "making theatre" of unspeakable frustration and despair – what might have been will never be, except in this performance. But there is an added dimension in the bizarre routines and speeches of Bea's mother and her Belfast-born carer, "Not Gay Ray".

Dresses are exchanged, other characters (the absconding father, for instance) obliterated, the world denied. Ray provides not only the possibility of sexual comfort, but speeches of Blanche DuBois and the pleasures of theatre-going with ice-cream and tabs of acid in the interval.

Perhaps Gordon doesn't nail this; the set-up runs out of puff. But this is a modern, socially concerned version of Jean Genet's The Maids, with its kinky role-playing and power bargains. Pippa Nixon's Bea, athletic and manipulative, has a blissful comic edge; one day she is making an earring, the next asking Ray to pay attention to her appetite down below.

There is a touch of fantastical oddness in Paula Wilcox as the barrister mother and, especially, Al Weaver as Ray, who dances to his own tune as much as to Bea's before being dismissed as if he were a hired hand. Mother and daughter are finally on the bed together, and there is some suitably poisonous music by Nick Lloyd Webber. The play is by no means perfect but it is unusual and disturbing and the enclosed world of the gigantic bedroom is well designed by Alice Woodward.

To 8 January (020 7478 0100)