Even though James Joyce's only play, Exiles, is unexpectedly traditional in form and structure, its psychologically nuanced portrait of a marriage in emotional distress manifests a startling modernity. Its ideas about sexual freedom and experiment within a traditionally closed marital framework remind us that some Edwardians were free radicals.

If it had not been for Hildegard Bechtler's dreamy, glass-walled evocation of a dusky, Edwardian drawing room in a Dublin suburb, her period costumes and the relative formality of speech and address, you could almost take the play as a 21st-century warning on that successor to the marital love triangle - the daunting figure of the erotic rectangle and how it comes to be drawn.

Completed in 1918, but not given a full-scale London production until Harold Pinter directed it in 1970, Exiles offers a barely disguised selfportrait of James Joyce the literary artist as a homoerotic masochist. The focus is upon a willed act of adultery.

Richard Rowan,the husband in Exiles, mysteriously encourages his wife, Bertha, into an extra-marital fling with his best friend, the journalist Robert Hand.

Rowan's motives and Bertha's true feelings remain tantalisingly shrouded in doubt and never elucidated. It is the attempt to set adultery in motion that gives the play its driving animus, with Richard kept informed by Bertha of each erotic move on a sexual journey supposed to end in the bedroom.

Peter McDonald's enthralling performance of becalmed, brooding intraversion ensures the husband keeps his inscrutability intact. At first, though, Rowan's intense encounter with Marcella Plunkett's attractively melancholic Beatrice, Robert's cousin and former fiancee, make it seem he wants Bertha bound for adultery so as to feel free to move in the same direction.

Yet in the Ibsenite and key scene of confrontation, Richard confesses a passionate longing to be betrayed by his best friend and wife, a confession that Adrian Dunbar's histrionic Robert seizes upon for his own sexual advantage. Dunbar, who looks too old to be playing Richard's contemporary, imposes a style of histrionic, comic bonhomie upon his character that fatally sabotages any sense of the men being bound by more than convenience.

MacDonald's production sacrifices Exiles' governing mood of perverse and agonised sexual confusion, which McDonald, Miss Plunkett and Dervla Kirwan's troubled Bertha powerfully transmit, in favour of Dunbar's misguided, winsome perkiness. James Joyce is accordingly done down.

• Until 26 October. Information: 020 7452 3000.