Innocent playgoers, who expect the theatre to stir their emotions and leave their minds on hold, beware! Rock 'N' Roll only fills the gaps between scenes.

Watching Tom Stoppard's extraordinary, epic drama of politics, persecution and protest in 20th-century Czechoslovakia, with Brian Cox's uncouth Marxist professor at Cambridge, passionately clinging to his Communist convictions while the Perestroika Revolution sidles up to him, is rather like struggling to answer a compulsory degree question in Advanced Stoppardian studies.

There we sit gazing at Robert Jones's dull, minimal revolve set. A garden scene and a book-lined study trundle into view and we are whisked in Trevor Nunn's impassioned production between troubled Prague and calmer Cambridge.

In the blackouts that separate the scenes, rock 'n' roll tracks, from Bob Dylan, the Velvet Underground and the Rolling Stones blast potent sounds into nostalgic ears.

How does Stoppard, in spanning the years 1968 to 1990, interrelate Czechoslovakia's Velvet Revolution, Anglo-American rock classics, the Czech band The Plastic People of the Universe, vanished Pink Floyd member Syd Barrett, etymological research into the lesbian poet Sappho and even the Materialist theory of consciousness? This last question looms and hovers.

The first, fascinating point to help towards understanding is that the hero, Rufus Sewell ' s superlatively incarnated Jan, a Czech philosophy student under the red wing of Cox's academic and mad about English rock'n' roll, is Stoppard's alter ego, a fantasy/imaginary version of himself, had he never left Czechoslovakia, aged two, in flight from Hitler.

So Rock 'N' Roll perhaps plays out a guide to how Stoppard would have shaped his political identity in the Seventies after Czechoslovakia had been invaded by Warsaw pact forces.

Opposing points of view are aired, though not with that great dialectical force between Cox's unduly furious, single-dimensional Professor, who refuses to jettison faith in the Revolution and Jan who on returning to Prague refuses to join his Havel-like friend, Ferdinand, among the dissidents.

It is only the arrest of the apolitical Plastic People of the Universe which fires Jan with rebellion. He sees the Plastics as pagans, claiming a fundamental right to be free of ideology's baggage and be true to their creative selves. Jan suffers imprisonment with stoicism and passion that Sewell makes memorably poignant.

This is the clue to the linkage of Stoppard's apparently disparate thematic elements. Beneath the surface of this capacious, insufficiently conflicted drama, there beats the Stoppardian romantic heart.

The materialist theory of brain function, according to Max's wife, Sinead Cusack's devastating, cancer-ridden Greek classicist, as she rips open her dress to show a ruined body, means nothing to her. She believes in souls, in minds not brains, in rare individuality of which Sappho's undeciphered texts are such difficult, classic examples.

Similarly Stoppard's relish for rock'n' roll, for the exiled Barrett seen as some crucial, romantic equivalent of the Great God Pan emphasises Stoppard's belief in the influence of mystic outsiders.

Rock 'N'Roll, in its humorous, domestic settling of scores, revelation of middle-aged love and concealed altruism, in Max and his daughter, drive the play on a more mundane tack. This is not quite first-rate Stoppard but still ought to generate a big and serious stir.