The Cut, Donmar, London

Is it sexual? Religious? Oh, who cares!

In Mark Ravenhill's new dark fantasy, Ian McKellen is sitting behind a desk - complete with silver-rimmed specs, moustache, three-piece suit. Set in an authoritarian state on the cusp of change, The Cut is a half-political, half-domestic drama about guilty consciences and purges. In Michael Grandage's production, a steel operating table gleams ominously in the centre of McKellen's office. He has an appointment with a patient, or is Jimmy Akingbola's John some kind of martyr/detainee? Dressed in reddish-orange and barefoot, John is keen to receive the Cut, yet McKellen is evasive, gabbing about new directives. In fact, he's having a breakdown, suddenly spewing four-letter words and talking about the agony this operation causes him - a job he conceals from his wife and boys.

Nonetheless, John insists that the instruments are fetched, calls the Cut a beautiful old ritual and makes the bureaucrat close his eyes, standing behind him like some priestly hypnotist, preaching about escaping into "liberty, freedom, nothingness".

We subsequently see McKellen at home with his plush wife (Deborah Findlay) who oscillates between small talk and needling suspicions. Then McKellen is in prison, rejecting offers of mercy from his fast-rising son (Tom Burke) who calls his father evil but insists the new order is humane.

Ravenhill keeps you in the dark about what exactly the Cut is meant to do or signify. He hints that it's an extermination programme related to race or class, but it could be a more symbolic psychological release, religious or sexual. Who knows and, I'm sorry to say, who cares? The cast mostly contrive to be enthralling with McKellen impressively reined in, flicking between the satirical and the serious, deceptive calm, rage and despair. But the dialogue is repetitiously padded and Ravenhill's dystopia rings hollow, portentous yet vague and heavily indebted to 1980s Pinter. Pity.

Booking to 1 April, 0870 060 6624