A Number, Crucible Studio, Sheffield <!-- none onestar twostar threestar fourstar fivestar -->

Caryl Churchill's A Number, which had its premiere at the Royal Court in 2002, is, quite simply, the best work for the stage written in the new millennium. In an act of profound imagination, it looks at the implications of cloning human beings. How does this procedure affect our idea of what it means to be an individual and what it means, in particular, to be a father or a son? And how might it alter our beliefs about the relative importance of nature and nurture?

Jonathan Munby's superlative revival of this two-hander in the Studio at Sheffield's Crucible Theatre boasts a casting coup. A play in which a father confronts, in a succession of separate encounters, three (of 21) cloned sons here stars the real-life father-son combination of Sam West (the Crucible's artistic director) and his pater, Timothy West.

The studio space is configured like a boxing ring with the audience seated on all four sides, and Paul Wills' brilliant design is surmounted by a glass chandelierconstructed from test tubes. Equipped with a London accent and a hunted look, Timothy West's excellent father is first quizzed by a sensitive, favourite son (Sam West in a tour de force of three thoroughly differentiated performances). Increasingly harrowed, this 35-year-old has discovered that he is one of a batch and the worry forms that he may not have been the original.

In the next scene, the touchy, tough and malignant 40-year-old prototype arrives. Gradually we realise that the father, left wifeless, screwed up with this model. The illusion fostered by cloning, that we can have complete second chances to rectify our errors, is exposed when the original murders the son who was created to replace him, the once innocent "first-born" who was damaged by his father's neglect.

A Number is not, in any straightforward sense, against cloning. Churchill's trump card is that, in the final encounter, after both the other sons are dead, the father tries to find a replacement in yet another clone, who turns out to be a slightly boring maths teacher who isn't afraid of being one of "a number". To the father, who equates parenthood with sharing a psychic wound, he's a grave disappointment. So the play is both pessimistic and optimistic, suggesting that there's a tragic inalterability in some people and also that, with sufficiently good nurture, others can escape from so-called genetic determinism.

To 11 November (0114-249 6000), then touring