A Number, Royal Court Theatre, London

Two men on a raked parquet floor, one in a baggy suit, the other in jeans and T-shirt. There's no scenery to speak of. There's no context and no background, but you can tell that one actor is old enough to be the other's father. In fact, it seems as though he is and then it quickly seems as though he might not be. Because the son – let's call him that – has just discovered that he is one of "a number" who were cloned. That word, the C word, does not occur even once in the play but that's the premise. So the question hangs: was he the original or a copy?

But this isn't a play about ethical quandaries, it's about the very basics of fatherhood. About the right to change yourself and the need to stay the same. And crucially, it's about lies and truth. Because we can only have got to scene one thanks to the father's long-term lies to the son who thought his mother died at birth. But of course this son now knows that was not so. So Father confesses: she was killed in a car crash along with their first child, a son. And the current son – I hope you're following – was cloned from him, mad scientist-style. Except that also turns out to be a lie.

Each stage of this play advances by unravelling the lies of the previous stage. There was no car crash. The first son is alive and he turns up, all brooding, rottweiler menace. Not surprising, after he was cruelly neglected by his drunken father and taken into care.

Not a bad premise, you'll be saying. Plenty of meat here, I can see this hotting up nicely. But the problem is that the setting, the dialogue and frankly everything else is so etiolated, so starved of texture and colour and humanity that it's hard to care. It's impossible. Yes, Michael Gambon and Daniel Craig (playing a full range of clones) give it everything they've got. But it's more a bravura acting display than anything approachably human.

Caryl Churchill has marked out territory as a writer who creates perfect half-sentences, unfinished ideas (all quite clearly demonstrated in the text like a musical score). But her technique is so perfect that you can see the actors, as though they are playing the most fiendish of Bach fugues, moving between these half lines, criss-crossing and swerving and stopping and starting until it all slots together perfectly. You start to feel that the whole thing is being delivered as a set of half-lines, neither of the players really showing any intent to complete the unfinished thought. In Stephen Daldry's production, there's no passion, just speed and finesse. It's magnificent but it's not a play.