Theatre: 20-52 Tricycle Theatre, London

TRUTH, they say, is stranger than fiction, a statement that would be more accurate if "truth" were replaced with "the facts." It's certainly true that the piece devised and mounted by Edinburgh's Grassmarket Project encouraged one to feel that the facts can beat fiction when it comes to making powerful theatre.

Under the directorship of Jeremy Weller, GMP is committed to giving people from under-represented groups, who have never acted before, the chance to tell their own stories on stage. The homeless, young offenders in prison, and mental illness as it affects women, have been some of the subjects tackled. The raw, emotional impact of these pieces tends to be devastating, though it would be self-delusion to claim that one's response is utterly free of prurience. What gives the shows their moral authority, however, is the way that first-hand experience is conveyed by people who have been put in control of the effect they are creating.

20-52, GMP's latest venture, provokes, on the other hand, an ease and distress that can't be entirely ascribed to the nature of the material. The title is taken from the "Self-harm At Risk Form" of HM prison services and the piece focuses on Stephanie Lightfoot-Bennett (played by herself), a woman obsessed with discovering the truth about her twin brother's brutal death in 1992 while in police custody. The horrifying details of the case and the scandalous way in which the system of investigation is biased in favour of the police quickly become subsidiary, though, to a dramatisation of the toll it has taken on Lightfoot-Bennett.

A combative, intelligent, unfoolable woman, she paints an unsparingly rebarbative portrait of herself. We see how the situation has turned her into a hard, fixated absolutist who dismissively resents anyone who attempts to share or trespass on her pain at the same time as rejecting, with a jeer, help that falls short of her own round-the-clock dedication. Rather than concentrate on her difficulties in winning the case (a third inquest looms), the piece diverts our attention to the way nobody can win with her.

Everyone is presented as though her understandably partial view of them told the whole story. A trendy, pony-tailed racial equality campaigner becomes a caricature of conceited self-interest. The main concern of her surviving brother, it appears, is that the other kids on the street are laughing at him for not taking the law into his own hands. The same male anxiety about looking a fool to friends seems to animate her macho, weight- training husband who is reduced to furious jealousy by her sarcastic inattention and the sexual threat he imagines is posed by the Racial Equality smoothie.

All of this has the effect of obscuring an important truth: that the injustice done to her brother would have been every bit as unjust, even if it hadn't destroyed her life into the bargain. The play evinces a meanness of spirit not before encountered in GMP shows. I sincerely hope Stephanie Lightfoot-Bennett achieves success at the third inquest, while feeling that, in its treatment of the people around her, 20-52 has itself not been above doctoring the evidence.

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