Atonement (15)

This momentous adaptation of Ian McEwan's Booker-nominated novel arrives with such fanfare and folderol that you can almost smell the engine of the studio's publicity machine overheating. I seem to have read more interviews and articles about the film than anything else this summer bar Amy Winehouse's drug problems.

The studio is right to think it is on to something, what with its two hot leads, a director whose 2005 debut, Pride & Prejudice, made a decent fist of the nation's most loved book, and the prestigious shadow cast by a novel some would rate as its author's very best.

The main challenge that adapters of McEwan's work must meet lies, essentially, in raising the dread. The director Joe Wright, working from a screenplay by Christopher Hampton, passes this test quite brilliantly, for the first hour at least. The ominous tone of the novel is immediately suggested in the opening credits, tapped out on the clicking keys of a Corona typewriter.

It is there also in the patterning of shadow and light across the Victorian Gothic mansion where the first part is set, on a sweltering summer's day in 1935. And we surely read some of the trouble to come in the pursed, watchful face of Briony Tallis (Saoirse Ronan), the 13-year-old daughter of the house's owners, a prissy miss who fights boredom by writing a play and dragooning her cousins into the staging of it.

It's too bad then that the cousins capriciously abandon the play and its outraged author. Briony, her adolescent head aswirl with half-understood desires and prejudices, seeks another subject to feed her imagination, and finds one with calamitous consequences. She spies her older sister Cecilia (Keira Knightley) in an argument with Robbie (James McAvoy), the son of the estate's housekeeper and, obeying her own storytelling instincts, she fatally misinterprets their relationship. There are echoes here of the Losey/Pinter adaptation of the LP Hartley novel, The Go-Between (1970), in which a child's interference (in another heat-struck English country house) spells doom for a pair of class-crossed lovers.

This, however, is much the stronger of the two, more delicate in its nuances and more vivid in its psychological complexity. The way that Wright replays two key scenes from different perspectives skilfully underscores the irony and heightens the tragedy. Ronan is superbly cast as mischief-making child whose innocence seems a kind of insanity, while Knightley and McAvoy are serviceable as swan and swain.

I've never been quite convinced by Knightley, and remain agnostic after this, but she looks the part to a brittle "T" and wears the clothes beautifully: the chartreuse evening dress is a bit of an event in itself. (Jacqueline Durran's costume designs are outstanding.)

On the margins, Briony's cousins, hilarious ginger twin boys with a sister who's Violet Elizabeth Bott to the life, are a visually witty delight, and as the confectionery heir Marshall, Benedict Cumberbatch mixes the suave with the sinister to brief but memorable effect: the "chocolate cocktail" he makes for Cecilia and her brother (Patrick Kennedy) is perhaps emblematic of the childhood/adulthood divide that the story keeps probing.

So compelling is this first section that what follows, while confidently handled by Wright and Hampton, seems a little underpowered in comparison. The action, shifting five years on to May 1940 and the BEF's inglorious retreat to Dunkirk, finds Robbie wounded but determined to make it back to England and to Cecilia.

The linking scene in a London tea-room where the lovers meet after their cruel interruption is a lovely miniature, a Brief Encounter of clipped tenderness and regret, but Robbie's exhausted footslog through northern France marks the point at which the film's choke-hold on us begins to loosen.

It's odd, because this middle section contains the most remarkable passages of McEwan's book, a fugue-like account of desperate survival amid strafing from German planes and the open-air slaughterhouse of the retreat. Wright does one terrific thing here, a flowing steadicam shot involving thousands of extras on the beach at Dunkirk (it was actually shot in Redcar) that encompasses the almost surreal chaos of an army routed and waiting for rescue; in the background you notice the Ferris wheel and the bombed-out buildings, in the foreground are drunken squaddies, cavalrymen shooting their horses and mechanics spiking their vehicles to render them useless.

It's bravura film-making, self-conscious perhaps, but impressive withal. Even the hymn that a chorus of soldiers sings on the ruined bandstand is carefully chosen – "Dear Lord and Father of Mankind", whose first line ("...forgive our foolish ways") chimes appropriately with the central theme.

This is meticulously done, though the remainder of the Dunkirk sequences and the switch to Briony's baptism of fire as a nurse in a London hospital pack a far smaller emotional punch than I recall from the novel. There are good things still: Romola Garai ably incarnates the 18-year-old Briony (and looks bravely plain in doing so) and Daniel Mays as Robbie's cockney companion on the road to Dunkirk has absolutely the right demeanour for the period. You feel there are many scenes and moments that Wright has taken pains over, and you admire him for it.

All the same, I feel unable to swell the wave of hysterical press notices that have been acclaiming the film as a masterpiece and an "instant classic" – whatever that is. The literary conceit that seals the story, with the ageing Briony's look back in anguish, is completely unconvincing, as it was in the novel. The explanation of how Briony (played now by Vanessa Redgrave) attempted to "atone" for the ruinous folly of that day in 1935 actually makes her look quite as self-absorbed as she was in her youth. "I gave them a happy ending..." she says, but the catharsis that the film has been building towards is conspicuously absent.

Let's be grateful for an intelligent, handsome adaptation of a McEwan novel – rare in itself – and praise a young director with visual ambition and a way with actors. But let's not burden him with the hopes of the British film industry just yet.