The film adaptation of 'Anna Karenina'

There's a gorgeous, tender declaration of love towards the end of Joe Wright's Anna Karenina, a sequence that may be the best thing this restless filmmaker has ever done - up there with Donald Sutherland's chuckling joy at the close of Pride & Prejudice. It involves none of the main three characters, but Levin and Kitty, a pair whose fumbled courtship other versions of Tolstoy's novel have often omitted.

It's also the film's simplest scene - almost wordless, as Tom Stoppard's script uses a child's alphabet blocks to heal this scarred relationship, spelling out what the paramours have to say to each other, but must. Domhnall Gleeson nails Levin's adorable self-seriousness without sentimentalising what can make him hard work, and Alicia Vikander, the lovely Swedish actress from A Royal Affair, modulates stunningly from flighty impulse to a chastened, trembling realisation of what love can mean.

Hidden gold, these are the film's two best performances, which is no huge slight on Keira Knightley: the more invective she seems to attract, the better she's actually getting. Her Anna is a headstrong, inescapably actressy creation, full of fussy vitality and verve. This is the third film she has made with Wright and it's clear she thrives under his direction. The emotional demands of this role bring out an intriguing combination of masculine certainty and bitter regret. If she never disappears entirely under the character's skin, that's more the fault of Wright's determinedly self-conscious choreography, which places such a stress on red-curtain artifice that it often threatens to upstage Tolstoy entirely.

Upstaging is exactly the risk, given that Wright's opening gambit is to house most of his movie, like that dream-ballet in The Red Shoes, inside the flexible space of an imagined theatre production. The first reel feels like a hectic dress rehearsal for what's to come, which is all very well, but key story points are submerged like Post-It Notes under a floor-length rug.

Imagining the rigid social whirl of St Petersburg as bustling, operatic live performance, Seamus McGarvey's camera is flung around, taking in dinner dates, bureaucratic ritual and every costume in Jacqueline Durran's capacious wardrobe. Actors depart from one set and reappear to have another erected behind them, in elaborate single takes that can't fail to impress; yet, as with that much-ballyhooed tracking shot around Dunkirk in Wright's Atonement, you wonder if their main function is simply to be noticed.

Wright's parents were puppeteers, which must have given him this unashamed love of the proscenium. But did it need to be so arch? By unleashing his inner Baz Luhrmann, he's made a film that's almost inevitably an acquired taste - a curate's Faberge egg. The energy of his big ideas can't silence a nagging hunch that they don't quite fit: isn't Anna swept, by all these phallically plunging pistons and toy trains, a little too inexorably to her self-chosen ruin?

When the movie gets less effortful, it improves. Tete a tetes with Jude Law's dour, upright Karenin, a figure waiting judgmentally on the movie's darker periphery, have a biting Stoppardian economy, and the debates between Levin and Matthew Macfadyen's impatient bon vivant Stiva - another choice turn - create a thematically satisfying counterpoint to the main event.

"If only" haunts the book as a recurring thought - if only Russian divorce laws were more liberal, for starters. It's hard to watch Wright's version without wondering what might have been, if only Knightley had a worthier scene partner than Aaron Taylor-Johnson. Vronsky may be wildly impetuous and lacking in self-knowledge, but he's never been this callow fool, twirling constantly in his daft white cavalry suit. When he opens his mouth, you prefer the pirouettes, but this is a rock-versus-hard-place alternative, and it damages our view of Anna - she's willing to sacrifice everything for this wooden twerp?

Wright tries out such virtuoso tricks he calls to mind Orson Welles's description of cinema as the biggest toy-train set a boy ever had. Like all the rest of us who aren't Orson Welles, he might want to tread more carefully at the platform edge.