The Cut, Donmar Warehouse, London <!-- none onestar twostar threestar fourstar fivestar -->

Ian McKellen and Mark Ravenhill are pioneering exponents of "posh panto"; Sir Ian has given us his Twankey and Ravenhill is due to pen the Barbican's first foray into the genre. But there's not a particle of panto spirit in The Cut, the darkly comic new play in which the actor and the trenchant author of Shopping and Fucking join forces.

Set in a sinister Orwellian state, Ravenhill's drama consists of three climactic episodes in the life of a civil servant, Paul, whose job is to administer the Cut, a ritual punishment inflicted on the subject classes. In a superbly sustained performance, McKellen begins by highlighting the clash between the punctilious functionary and the man rotting inside with guilt.

Paul is confronted in his gleaming office-cum-operating-theatre by a zealot, John (Jimmy Akingbola), who craves the barbaric procedure as an ancestral badge of pride. This creates a ludicrous situation in which the suicidal torturer pleads for his victim to shoot him before reluctantly performing the incision.

"At the end of the day, I'm a good man," says Paul during the awkward dinner he eats at home with his spouse (Deborah Findlay) in the second episode. Like many remarks here, it has a disturbing reverberation; and before the end of the day? Findlay beautifully lets us see how the apparently petty, spoilt wife has suspected the real nature of his job for quite a while. It's hard to disentangle the selfish and the selfless in Paul's decision to carry the burden of the secret alone. His marriage is withered and sexless. But then, he wanted to protect his family and the instinct for trust would atrophy in his line of work.

Michael Grandage's production adroitly balances the play's pain and its absurd comedy, and McKellen's portrayal keeps Paul valuably elusive throughout. In the final scene, he's a political prisoner, visited by his son (Tom Burke), a humourless new broom of an incoming regime. When Paul desires brutal punishment, is it true remorse or a satiric spanner selfishly thrown in the new works? The Cut is designed to leave such questions nagging.

To 1 April (020-7240 4882)