Sokurov has found a way to meditate on the nature of Russian national identity in its post-Soviet age

This is sort of an oddity - and sort of a miracle. It's a strategic and logistical achievement whose effect, over an hour and a half, builds into something weirdly moving and certainly unique.

The prolific Russian director Aleksandr Sokurov has composed a cinematic love letter to the Hermitage museum in St Petersburg, the "ark" in which Russia's elite cultural identity has been preserved more or less inviolate from the calamities of the 20th century.

He has done this by contriving a kind of installation-exhibit, something between cinema and theatre. His camera follows an imaginary French marquis who ambles questioningly through the museum, hardly realising why he is there or where he is going. Sokurov's own voiceover is heard in a whisperingly Beckettian interior monologue, also unable to credit what is going on.

Everything unspools in one single, unbroken travelling shot, moving sinuously around the museum, roaming down corridors, nosing into chambers, peering up and down stairwells - encountering scenes from Russian history from the 17th to the early pre-Revolutionary 20th centuries: from Peter the Great to Nicholas.

It is acted out by battalions of players and musicians in full costume. And all of it seamless; not a single cut or edit. Cinematographer Tilman Büttner had to carry a specially modified Steadicam capable of recording up to 100 minutes of high-definition video on to a hard disk.

Russian Ark is a fluid dream-epic with no special effects. What you see is what you get. If there had been a single mistake, if someone had fallen over or if a door was jammed - or if the camera had blundered across a mirror or reflective surface - then Sokurov and his army of actors would have had to go right back to first positions.

But although Sokurov was many months in rehearsal, he was only allowed to film in the Hermitage for four hours on a single day. So it's arguable if he would have been permitted even a second chance. This is high-wire movie-making.

As it happens, there are errors. Extras glance uncertainly at the camera, and at one stage a bewigged gentleman, bent double, is seen scampering out of shot. But this only heightens the sense of witnessing an extraordinary, one-off event: a ballet in which the camera itself is the principal dancer, never off stage.

There really is nothing comparable to this "single-breath" cinematography. Mike Figgis's Timecode, on video, can't match it for length and, of course, Hitchcock's Rope had to find areas of complete blackness to hide the joins. The closest parallel I can think of is Olivier's Hamlet from 1948, in which the camera had a habit of roaming between scenes in the hugger-mugger corridors of Elsinore Castle to seek out the drama; there was a notable reluctance to cut and a similar theatrical sense of unbroken space.

But why do it in the first place? Why abandon the grammar of cinema? The edit is what gives the film-maker the ability, in Tarkovsky's phrase, to sculpt time - and space too. Audiences can hardly be expected to admire the technological achievement alone; so is it just Dr Johnson's dog walking on its hind legs?

The answer lies in the real time/real space continuum established by these working methods, allowing one to experience the real presence of a giant cabinet of treasures that Sokurov has assembled in this historic place.

If, in another sort of film, we see the exterior of a house and then cut to its interior - well, perhaps that is what the house really looks like inside, or perhaps they've filmed an entirely different one. Spatial reality is a fictional construct.

But this is a very different sort of artefact, one built and choreographed and enacted in the physical world, and the resulting film is a record, rather than an invention, of what happened over 90 minutes on a single day. We can witness it everywhere in a way a theatre audience can't.

It has its eccentricities and longueurs, undoubtedly, but even these add to its savour.

Sokurov has found a way to meditate on the nature of Russian national identity in its post-Soviet age, and rediscovered its long-forgotten passionate identification with the European civilisation of the west, between the Enlightenment and 1917.

The implications of this rediscovery are complex. Should this splendour be celebrated, but kept under glass - or made to live again?

At any rate, Sokurov's marquis cannot visit any scenes from Russia's modern era. It's like the fictional passage Peter Ackroyd put into his Charles Dickens biography, imagining Dickens strolling in the Geffrye museum in east London; he is at home in the mid-Victorian room but shrinks from entering the 1930s room. Somehow, he cannot cross the threshold and walk on its linoleum flooring.

Not all films can be like Russian Ark, of course, and it's debatable if they could or should. But the effect of this cine-theatre event is extraordinary. When the cast of thousands are finally gathered for a vast, valedictory procession down the main staircase, and the marquis moans: "Farewell Europe" - it is desperately sad.

Sokurov is hardly as well known here as he should be. Moloch, his superb film about Adolf Hitler, has never been released in the UK - baffling, as nazism is such good box office. But Russian Ark is enough to be going on with.

Could a British or American or French director have had the imagination and determination to achieve a similar costumed movie-staging in Buckingham Palace or the White House or Versailles? Sokurov did, and he has conjured up a Midsummer Night's Dream of Russian history.