Caryl Churchill's plays are often a mystery, sooner to be puzzled over than necessarily solved. The most formally adventurous of our playwrights, she stands next to Harold Pinter in the meticulous sense of otherness she creates around her characters, where what they say is not necessarily what you get, or means what they say. And as often in Pinter, there are tense, terse family relationships at play.

In her latest piece, A Number, comprising five short scenes in less than an hour's running time, we observe the encounters between two men: an older man in a crumpled suit, a younger man in white t-shirt and jeans. The younger man is his son. Or rather, sons.

But Daniel Craig, playing them, doesn't differentiate in dress and only in accent for the third of them; meanwhile, you have to figure this out for yourself (on press night, they weren't selling copies of the programme script until after the show, so you would have had to grasp this entirely for yourself).

Skip this paragraph if you want to maintain the complete air of cloaked mystery, but it turns out that the boys have been cut from the same cloth, so to speak. Not merely do they share the same father but, thanks to some unspecified act of genetic engineering, they have been cloned of each other. Now, separately, they're meeting papa again.

As the father is played with a brooding sense of pain and unresolved emotions by the always utterly compelling Michael Gambon, there's an irresistible fascination to the interplay between him and his sons, two of them conflicted, one uncomplicated. Craig - currently to be found in the role of another pivotal son in Sam Mendes' film The Road to Perdition - is terrific, too, in his several roles.

Stephen Daldry's stark, spare production - played out on a parquet-floor platform that's bare except for two non-descript chairs and a small table - distils a short, strange play into a compelling one.

- Mark Shenton