In Cloud Nine, Caryl Churchill does not just make glorious fun of Anglo-Saxon guilt about the naughtiness of erotic acts outside the sacred realms of marriage. She also offers a poignant, psychologically astute guide to the way the Victorians' prohibitions on sex were challenged 70 years later by dare-devil outriders in the Gay and women's liberation movements: at last people dared shape their sexual identities and escape the chains of decorum.

Churchill's first assault on conventional morality is staged in late 19th century Africa. Grandiose colonial administrator Clive, over-projected by bellowing James Fleet, presides over a family in crisis, more than a little touched by adultery, homosexuality and a lesbianleaning governess.

To lay dramatic emphasis upon the mood of prevailing anxieties about sexual identity and desire, Clive's wife Betty is played - brilliantly - by a dragged-up man (Bo Poraj). The effeminate son Edward is portrayed with similar conviction by a woman (Nicola Water). These Empire Builders live in fear not only of themselves but also of the revolting natives. "If it comes to being killed I shall take it as calmly as anyone," says Joanna Scanlan's wonderful, stiff-lipped, repressed mother-in-law.

"I suppose getting married wouldn't be any worse then killing myself," observes Tobias Menzies's suave, self-loathing, gay explorer, Harry, already come to sexual grips with Edward.

In scenes of dazzling verbal comedy, enhanced by the superlative, versatile ensemble of actors, Churchill suggests how Victorian diction was designed to hide rather than express feelings. It is the author's imaginative conceit that the second act, set in 1979 London, should be just 25 years later, with the Irish problem casting late imperial shadows.

Churchill conjures a dreamy, radical sexual scenario, little helped by designer Peter McKintosh's bare, unevocative staging. Clive's unhappily married daughter, Victoria, falls for the lesbian Lin, while domesticity-loving gay Edward loses his boyfriend and joins the pair in an adventurous ménage à trois.

Their mother, Betty, now played by a woman (a transformed Nicola Walker) as if to suggest her newly acquired selfconfidence, leaves her husband and settles for an independent life. In the emotionally devastating fantasy sequence of the finale Betty embraces and forgives the ghost of her younger self.

Cloud Nine, despite Thea Sharrock's far too austere and deromanticised production, ought now to be established as one of the great psycho-sexual comedies of the 20th century.

• Until 8 December (020 7359 4404).