Michael Frayn's play about the chancellorship of Willy Brandt, whose leadership and attempts to forge a relationship with the East were brought down by the discovery that one of his most trusted party workers was an East German spy, sounds dry as dust. It is the sort of play, full of grey men in grey suits doing lots of talking about a subject - German coalition politics of the 1970s - that is not close to my heart and seems likely to make me wriggle in my seat and bite my nails.
Well, the nails remain intact, because Frayn's play is not only the most intelligent and wry in London, it is also the most moving. He deals with so-called challenging subjects yet has the rare gift of never patronising or flattering his audience. When Gunter Guillaume talks wistfully of missing the grey streets of East Germany it is not just funny but true.
Like Guillaume, the oily Stasi spy who eased himself into Brandt's inner sanctum, this play is deceptive. It is always what it appears on the surface but also an echo of itself. Like the Judas Guillaume, who loved Brandt and yet also betrayed him and who sacrificed his life to the mirage that was the East German state, this is a drama founded on double associations.
Its twin pillars are Brandt and Guillaume - men living in a divided Germany who took different routes and yet who end the play with the sound of the dismantling of the Berlin Wall ringing in their ears. The intimacy between Roger Allam's magnificent Brandt and Conleth Hill's equally compelling Guillaume is played to teasing perfection.
This is a play so sophisticated, so at ease with itself (at least it is after the first clumsy 15 minutes when Frayn has to feed us much crucial information) that you often feel as if you are watching several plays at the same time shifting and refracting off each other.
The mechanisms of political intrigue spring from the situation at the time, yet could equally apply to New Labour. Brandt's ousting offers a lesson to Tony Blair: that in politics it is always worth remembering that it is those closest to you that you should fear the most.
· Until October 16. Box office: 0870 0606633.
