Are the novels of Ian McEwan unfilmable? I ask only because, with the exception of The Cement Garden, his work has a habit of curdling en route from page to screen. What a strain it was enduring Enduring Love, and as for The Innocent - well, all I can say is that the film-makers should have changed their plea. An adaptation of the highly regarded Atonement doesn't break the McEwan curse, but it offers some clues as to why this unlucky streak persists.

The story hinges on a train of misunderstandings that goes off the rails spectacularly. Precocious 13-year-old Briony Tallis (Saoirse Ronan) is whiling away the summer of 1935 writing plays at her parents' country mansion, but her imagination spins out of control when she witnesses fragments of a real-life drama. What sort of mysterious friendship is her sister Cecilia (Keira Knightley) involved in with the housekeeper's son, Robbie Turner (James McAvoy)? And why does Cecilia plunge into the fountain, apparently at his behest - apart from the obvious reason that a shot of Keira Knightley in a dripping-wet negligee is unlikely to pinch at the box office?

Briony's fantasies are given the corroboration they need when she reads the steamy contents of a letter Robbie has written to Cecilia. What she thinks she saw leads eventually to the police being called; the rest of the picture traces the repercussions of this childhood misreading on the lives of all concerned as Cecilia and Briony (played as an adult by Romola Garai) become nurses and Robbie is despatched to war.

The incompatibility of McEwan's writing style with film grammar becomes apparent even before you realise that no attempt has been made to find an equivalent for the novel's central conceit, which is that Briony has written the very story that is unfolding. Whereas a portentous word or phrase can be absorbed in McEwan's text like a landmine, ready to be detonated later, the director Joe Wright goes straight for explosive effects. There's the shot of Robbie in the bath, gazing up through the skylight as a military plane passes overhead; that aircraft should be trailing a banner that reads "Harbinger of doom". Or there's the adult Briony loosening the bandages around a dying soldier's head. It is staged like something out of a Roger Corman B-movie, and as Briony unwraps her gory parcel, you realise the whole film is structured as a series of titillating, disreputable gifts to the audience.

Nowhere is this more apparent than in the technically accomplished but morally dubious scene of Robbie staggering around the beach at Dunkirk. Wright captures the episode in one protracted steadicam shot that accompanies Robbie through the debris as soldiers on horseback or in various states of injury cross his path. Whatever justification there might be for this shot is outweighed by its inherent boastfulness. The most effective unbroken shots pull you into the action so you forget there's even a camera there, let alone how long it has been running. In Atonement, the opposite is true: Wright brings aesthetic choice down to the level of a pissing contest - which would be regrettable in any context, but looks downright distasteful against the backdrop of Dunkirk.

One thing you can say for the shot is that it's consistent with the rest of the film in its mood of predetermined slickness. With its French Lieutenant's Woman-style close-ups of Cecilia and its air of inbuilt, David Lean-esque prestige, Atonement is rigged to be a Bafta magnet. Romola Garai and Benedict Cumberbatch, who has a chilling cameo as a predatory spiv, contribute brilliant work. But who among us can watch Keira Knightley without asking: wasn't Emily Blunt available instead? Knightley is able enough, but she never escapes the straitjacket of her own celebrity persona the way great actors do.

That's the Working Title formula for you: snap up a bestselling novel, ladle on the stars and the gloss, and watch the bucks roll in. But who is going to atone for Atonement?

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