Measure For Measure, Theatre Royal, Bath <!-- none onestar twostar fourstar fivestar -->

Things look pretty black in Vienna, even after the detested Duke absents himself and leaves in his stead Angelo, the deputy with ice water in his veins and, if the ribald Lucio is to be believed, elsewhere as well. The panels of a cold, dark, shiny set slide open to reveal only bars and more bars, in a city where the court is part prison, part nunnery, and the all-black, doily-collared dress of the men suggests a city where fornicators, professional or amateur, are already isolated outsiders.

Lucio says he speaks with the authority of one who has watched Angelo relieving himself, but if we recall that he also crisply remarks: "I know what I know," we realise that this vainglorious phrase might be the keynote of a play in which no one can rely on his ostensible knowledge, least of all the deputy. Certain that he is invulnerable to the passions that lead men to the lawbreaking for which he whips and hangs them, Angelo is struck, lightning-like, by love for Isabella. Then, believing he is possessing her, he shares a bed with Mariana of the moated grange. Lucio, certain that he is speaking to a friar of little account, defames the Duke to the disguised nobleman.

Only Isabella remains steadfastly herself, defying familial bonds and state authority as she refuses to yield her virginity to the besotted Angelo in return for the life of her condemned brother, Claudio. In the play's last moments, however, her world is turned upside down by an unexpected proposal that will make her plan for the convent and her limited self-knowledge as irrelevant as if she had traded herself after all.

Peter Hall's production at the Bath Festival brings out the ironies of this sardonic play - the scene in which Lucio, urging the young postulant to plead more and more fervently with Angelo, is clearly a version of a procurer's demands for more animation from an apprentice whore. But the scene, like all of those in which Andrea Riseborough appears, suffers from this actor's pinched, parched manner (in which virtue is imagined as stony-heartedness), her schoolgirlish voice and her constant traffic-signalling of her emotions with banal, choppy gestures. It is unlikely that the ferociously austere Angelo would succumb to a paler version of himself.

If only, one wishes (I always do, whenever I see this play), the actress in this part could be, while aloof, rapturously sensual, devoted body and soul to God. (It's not just the coincidence of names that makes me long for a young Isabella Rossellini.) One could then understand Angelo - a man who, fundamentally weak, is obsessed with appearing strong - subjugated by an Isabella whose sexual nature is expressed in powerful self-containment rather than the looseness that ordinarily repels him.

Richard Dormer makes a good shot at Angelo's anguish and despair, nearly in tears at his helplessness and confusion, but the lack of the sensual element in him as well makes his distress seem that of a man whose business deal has gone badly wrong.

It's a pleasure to hear James Laurenson's beautiful voice and his skilful way with verse (Barry Stanton, as the sonorous councillor, is also a pleasure to hear). But one is unsure whether the characterisation is simply shaky, or whether he means to suggest that the Duke, who seems moribund in his opening scene but is considerably livelier in his monkish disguise, has donned the cowl in an attempt to get closer to his people in the personal as well as the political sense, to stir the blood in his own sluggish veins.

In the final, lengthy scene of revelation and judgement - which itself could use more movement and spirit - his sudden proposal does not seem the end of a quest for wisdom so much as another desperate, ill-judged effort at humanity. Another troubling aspect of the Duke's last decision is the disparity in age between him and Isabella.

On the whole, in fact, this cast is rather long in the tooth. Ben Turner's Claudio is fresh-faced and appealing, but Michael Mears as his roistering companion Lucio looks old enough to be his father - and certainly old enough to know better, a fact that shadows an otherwise deft and witty performance with unsavoury gloom. Teddy Kempner's strenuously ineffectual comic turns as the bawd reflect his general sheepish manner, which suggests a man long retired from his trade rather than one so busy and thriving as to merit prison.

In the small part of the down-in-the-mouth Elbow, however, Edward Bennett succeeds in making the constable, whose demeanour is as hangdog as his moustache, hilariously lugubrious.

To 12 August, 01225 448 844; www.theatreroyal.org.uk