Those who feared that Ian McEwan's best-selling novel would prove unfilmable will be relieved to know that, even when adopting the basic structure of the book rather than going for straightforward narrative, Joe Wright has proved them wrong. The Venice opener, to be shown in London next week, is already being talked of as a triumph.

Christopher Hampton's screenplay, inevitably cutting down McEwan's 130,000 words to 25,000, still retains both the emotional power of the original and the central idea that one false move in youth can alter several lives for ever. It is a "big" film, set as lavishly as a none too huge budget will allow in England and France. But its ability still to seem an intimate portrait of three intertwining lives is its ultimate triumph.

The central character, if there is one, is fledgling writer Briony (Saoirse Ronan when young, Romola Garai and Vanessa Redgrave when older) who lives with her family in a Victorian Gothic mansion on the eve of the Second World War. Through a series of catastrophic misunderstandings, she accuses Robbie (James McAvoy), the housekeeper's son and the lover of her sister Cecilia (Keira Knightley) of a crime he did not commit, thus dramatically altering the course of three lives.

We follow, sometimes in turn and sometimes with the characters in tandem, how desperate the situation becomes, with Briony's guilt matched by Briony and Cecilia's search for Robbie when he comes out of prison and volunteers for service in the killing fields of France.

There are times, because the narrative is fractured, when we wonder who exactly is at the emotional hub of the film. Is it Briony or Robbie? It can't be Cecilia since Knightley, though by no means unsuccessful in the part, seems to have less of Hampton and Wright's attention than either.

But McAvoy's Robbie as the wounded lover is the best portrait he has yet given us, showing us a new maturity and a determination to get at the truth of his character, and Garai almost matches him.We get the feeling from these three actors that life is not a matter of a single truth but can be seen from several differing points of view.

Wright's summation of the havoc of Dunkirk suffers a little from a lack of financial resources but otherwise his telling of a complicated and complicating story has very few false notes. The film is totally different from an epic romance from Hollywood, and may surprise some who expect a British parallel. But, in essentials, it is all the better for that.