One of the last roles that Roger Allam played at the National Theatre was Adolf Hitler. He reappears now as Willy Brandt. This is tantamount to being cast successively as Satan and the Messiah. Brandt, who became Chancellor of the Federal Republic in 1969, did much to reverse the Führer's monstrous legacy, and his Ostpolitik was designed to promote peace in Europe by recognising, and regularising relations with, communist East Germany. Winning the Nobel prize in 1971, he was brought down by a scandal three years later when Günter Guillaume, a close personal aide, was unmasked as a Stasi spy.
Democracy, Michael Frayn's complex and richly rewarding new play, does not keep you in suspense about this agent's true colours. Quite the opposite. It's plain from the start and as he performs his (increasingly intimate) duties in Brandt's service, Conleth Hill's superb Guillaume - a troubled man posing as an unctuous nonentity - rattles out a running commentary to his East German controller, Kretschmann, who sits at a table as though directly observing. Thanks to this fluid, fertile comic device, the presentation of the West German government is infiltrated by irony from the outset.
The play is wonderfully alert to the piquant paradoxes and ironic twists of this intensely tricky period in Germany's conversation with itself. Initially distrusting the sincerity of Brandt's Ostpolitik, the East Germans wind up more intent on keeping the Chancellor in power than many of his own resentful SDP colleagues. As sketched here, post-war West Germany is a world where power depends on unstable, ill-natured coalitions and a bizarre partner-swapping game where an old ex-Communist can find himself forced into bed politically with an elderly former Nazi. Only Brandt, who fled the Gestapo and worked for the resistance in Scandinavia, is untainted by the war - though, to some, clean hands are themselves suspect.
As the Chancellor, Roger Allam gives a highly impressive performance, capturing the man's magnetism, his constitutional melancholy and a strange sense that the idolised public figure is a mask with no one behind it. On a split-level office set populated exclusively by men in suits, Michael Blakemore's excellent production controls the rapidly shifting moods with great sensitivity, particularly in those uncomfortable meditative moments when politician and spy, symbols of Germany's ideological divide, are seen to have haunting affinities. The pressures of history have left both with a strong feeling of self-alienation. Forced to assume various identities when on the run from the Gestapo, Brandt now describes himself as a suitcase with a series of false bottoms, unable to reconnect with the boy he once was. For Guillaume, a brief trip back to the GDR was like "eavesdropping on my own absence".
Even in their downfalls, these fatherless, womanising males are akin. Brandt fatalistically resigns when, innocent of duplicity, he could have survived. On arrest, Guillaume instantly confesses his guilt, thereby betraying his own political masters and giving the West Germans the watertight case against him they would not otherwise have. But while it irks the spy that he has been used by the GDR, it distresses him greatly that his case is exploited as a way of unseating Brandt, the man he has come to revere.
At the end, the Wall has come down, and Guillaume is just a face in the crowd that is cheering the former Chancellor on his tour. A single nation once more; but, tragically for the ex-spy, never again the double act that ironically pulled together in a time of division. Shaping a huge mass of material into intellectually stimulating patterns, Democracy offers a great deal more than a crash course in recent German politics.
To 30 December (020-7452 3000)