The Unthinkable, Crucible Studio, Sheffield

The playwright Steve Waters has a refreshing willingness to embrace contemporary life. In World Music it was the effect of the complex relationship between Africa and Europe on successive generations. Now, in The Unthinkable, he has returned to the the political backdrop of a previous play, English Journey, to present a well-defined picture of the progress of the five founder members of the radical think-tank Polis.

In Josie Rourke's adept production, on the enclosed bare stage of the Crucible Studio, their lives unfold, through flashback, in a whirlwind of words, acutely observed character portrayals and tensely ambiguous relationships. Since their Oxford student days, when plans were feverishly hatched and their youthful visions and ambitions took root, much has happened. Now they've come together, "idealists reunited", to celebrate the 15th anniversary of the founding of Polis.

From their first, tentative meeting in 1984, and then, five years on, when Polis becomes a reality, there's a palpable excitement as the group members gradually realise that they can think the unthinkable. Added to the volatile mix is Patrick, the manipulative MP and New Labour apparatchik, a role Nigel Cooke plays for all its worth, bringing to it the look of Jeffrey Archer and the speech mannerisms of Peter Mandelson.

In 2004, as international movers and shakers gather at the Polis headquarters in a remote Derbyshire farmhouse in the shadow of Kinder Scout, a public demonstration against capitalism is on the move outside, rumbling ever more loudly. But that's not the only cloud on the horizon. Public life impinges on private, past catches up with present, people have changed, and the anonymous circulation of documents relating to the group's past unsettles them all, threatening to unseat Ollie, engagingly played by Jo Stone-Fewings, now the new MP for South Yorkshire.

Well performed and neatly topical, The Unthinkable embraces many big ideas, global and local, philosophical and political, as well as personal. As a vision of how our media-fostered politics and under-scrutinised public life work, it's uncomfortably tenable. The women create the more interesting, revelatory characters: Katherine Parkinson, the strident activist Fran, whose concern for human rights and justice for all leads her into dangerous territory, and Susannah Wise as the clever, working-class Maggie, the straight-talking head of Polis. The male roles, though less well defined, are coloured with ingenuity and wit.

Waters writes with particular perceptiveness about the complexities of remaining true to one's beliefs in the reality of the unreal political world. However, perhaps because of the number of issues raised and the head- spinning speed with which they're tossed around, unanswered questions are left hanging in the air.

To 13 November (0114-249 6000)