THERE are no dramatists for whom I'd rather risk deep-vein thrombosis than Tom Stoppard, but his nine-hour flight through mid-19th-century Russian history isn't the easiest ride. Yes, The Coast of Utopia is refreshingly ambitious in its sweep. Yes, it's packed with reflections on idealism and political change that still have clout today. But the trilogy has its longueurs, its dips of energy, its relentlessly protracted arguments -and only sporadically the fun that is Stoppard's trademark.

The National suggests that any one of the evening's three plays may be seen in isolation. That's a dubious claim, since the pioneering socialist Alexander Herzen has a major role in the opener, Voyage, and goes on to dominate Shipwreck and Salvage. Miss part of Trevor Nunn's production, and you'll miss a large part of its main character. But if you want to sample a single play, I think the finest is Voyage, which starts in a genially Chekhovian style, introduces key characters, and gives you a sense of the intellectual hurly-burly of an age in which dissident aristocrats or "repentant gentry" were leading the opposition to a serf-owning society and a monstrously oppressive Tsar.

No fewer than four sisters are bubbling about grouchy, conventional old Alexander Bakunin's mansion, the sweetest of whom yearningly cries "Moscow!" at one point, but it's their brother, Douglas Henshall's nicely observed Michael, who is the centre of everyone's attention, especially his own. Stoppard charts his progress from a dizzy belief that "the life of the spirit is the only real life" to a conviction that this spirit has social obligations and eventually to a murderous radicalism, never letting you overlook the man's awful self-absorption.

He's compared with Guy Henry's Turgenev, whose dandified view is that "the only thing that will save Russia is European culture transmitted by people like us", and Will Keen's Belinsky, a critic whose roots are more proletarian than the others. Stuttering and hiccuping out his outrage -terrific acting here -he believes that "Russian literature alone can redeem our honour" and has the pluck to return from exile to proclaim this in the lion's mouth. But with the arrival of Stephen Dillane's Herzen, the trilogy finds its most articulate voice and, in so far as Stoppard deals in heroism, its hero.

Voyage largely involves the evolution of Russian dissidents from navel-gazing to commitment, Shipwreck the question of how change is to be achieved: from above or below or both? But before long the evening becomes a biopic involving Herzen, his exile to Paris, Nice and London, his post-1848 ennui, his wife and son's deaths, his launching of a samizdat magazine, his affair with his best friend's wife, his rejection by the young revolutionaries he helped to create.

All this comes with much speechifying -Herzen would be playing the earnest raisonneur in the middle of Armageddon -whose content is, however, hard to sum up. Let's say that, unlike Paul Ritter's dogmatic Marx but maybe like Stoppard himself, he believes that a mix of chance and choice determines social change. There are no rigid entities called "history", "the future" or "Utopia": just a duty to push the world forward as peacefully as possible, hoping it doesn't terminally implode as it goes.

Does Dillane's dry, inward performance give full life to this famously charismatic man? I wonder. I also wonder if Stoppard's references to, say, the Decembrists, Hegelian philosophy and Turgenev's anti-hero Bazarov will puzzle the uninitiated. At times I felt that too much research was being conscientiously crammed into the trilogy. And though the hifalutin, touchingly innocent talk about love has its place in a play about Utopianism, it leads to some loose, meandering episodes, as do the thematically similar but distracting ruminations on education. Yet this allows Eve Best to give the last of three super performances as the Herzens' pernickety governess. There's strong acting, too, from John Carlisle, Simon Day, Sam Troughton as a comically posturing Slavophile, and others.

Nunn brings his usual skill to a thickly peopled stage circled by lavish film-projections of Paris streets, Nice shoreline, Russian countryside, whatever; but he could have trimmed and tightened a bit. Still, when Dillane's Herzen ends up denouncing Utopian theory as a "Moloch that promises everything will be beautiful after we are dead", and acknowledges "the summer lightning of personal happiness", Stoppard's piece does sing. You leave it sated, exhausted, impressed.

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