Twelve years of Hitler. Four years of military government. Twenty years of conservatism and cold war. And now a hope at last of ending the long stalemate in Europe.” With Germanic precision, Michael Frayn’s new play starts at 11.22am on October 21, 1969, the exact moment Willy Brandt was elected chancellor of the Federal Republic of Germany, the first leftist leader in more than 40 years. Brandt’s rule seemed like Germany’s very own Camelot, a golden opportunity to come to terms with the second world war and, most significantly, to reconcile with the East German state that lowered alongside it.

His fall, when it came, was crushing: Günter Guillaume, his personal assistant, was unmasked as an East German agent, triggering a scandal that resulted in Brandt’s resignation when it was feared Guillaume’s revelations might include blackmail-friendly information on his impressive list of extramarital exploits.

It is rumoured there is more paperwork recording German life in this period than in the whole of the rest of the country’s history put together. Democracy could have been as gloomy and confusing as an old archive. Yet, just as Copenhagen broke down events of global magnitude into the atoms of human nature, Democracy (National/ Cottesloe) cuts through the history to focus on personal motivation. At times it comes across as The West Wing meets the Eastern Bloc: hugely entertaining, packed with verbal parrying and effortless wit.

What at first appears to be a dispiriting cast of grey, middle-aged men in grey, middle-aged suits quickly separates out into vividly imagined portraits. Roger Allam makes a painfully convincing Brandt, the volatile visionary on the one hand, the Julio Iglesias of the Bundestag on the other. The excellent David Ryall plays the Machiavellian Herbert Wehner, too long in the poison-tipped tooth to worry about the niceties of human interaction, while Glyn Grain plays Helmut Schmidt as Sir Humphrey with a freshly sharpened axe behind his back. Frayn captures the bitchy wit that often festers in Establishment men and proves that, yes, coalition politics can be fun.

Yet the real centre of power, in the play and in the government, is Guillaume, the “nobody”, strikingly conveyed by Conleth Hill. Guillaume comments on events to his handler, Arno Kretschmann (the fittingly pallid Steven Pacey), seated on the stage throughout as a visual reminder of successful infiltration. This is as far as the Spooks- style shadows stretch; the cutaway set is a model of modern Euro- efficiency, only the rows of paper-filled pigeonholes indicating hidden intrigues.

Frayn has the luxury of transparency with his characters, too, and his depiction of Guillaume, a servant devoted to two masters, is an empathic masterstroke. Hill is surprisingly camp, Uriah-humble yet acid-tongued, besotted with Brandt yet irritating enough to be borrowing a stapler in Ricky Gervais’s office. There’s a touch of Austin Powers, too: when he delivers an especially sly aside to Kretschmann, you expect him to drawl “Ooh, behave!” and head up a Stasi chorus line. It should seem off-key, but with Hill’s subtle shadings, it makes perfect sense, the servile softness disparaged by his employers complementing the theatrical flair of a pro- fessional liar.

For Democracy is obsessed with the roles people play, the choices they make and the divisions those choices leave behind. Brandt fixates on the lives he might have led, while the unlikely similarities between the chancellor and his increasingly devoted assistant are gently stressed. “How can you see into someone’s heart if you don’t fall a little in love with them?” says Guillaume when Kretschmann queries his affection for “the chief”. Frayn might be criticised for giving a traitor such warmth, but the playwright’s interest in people, in what it is to be human, means he, too, is a little in love with these characters.

Democracy is an educational play, certainly, filling in a part of history that usually has the allure of a post-war concrete building, yet Frayn also tries to fill in some of the mysteries that lurk within us all. “So many people with so many different views and so many different voices,” says Brandt. “And inside each of us, so many more people still, all struggling to be heard.” By laying this imaginative wiretap into the lives of Willy Brandt and the spy who loved him, Frayn obtains some valuable secrets of the heart.

Hear Michael Frayn talking about his play on September’s The Month CD-Rom