Read The Sunday Times review of Atonement

One of the great hopes of British cinema has opened the 64th edition of the Venice Film Festival with a bold assault on Ian McEwan’s novel Atonement. Joe Wright, a tender 35, is the youngest director to open this prestigious event. But the buzz, Oscar and otherwise, around this world premiere of Atonement is only partially merited.

Wright has assembled a catwalk of fashionable talent to reveal how a family is destroyed by a single lie. The film, adapted from McEwan's novel by Christopher Hampton, begins on a hot summer’s day in 1935. A country-house beauty, Cecilia (played by Keira Knightley), is having a secret affair with the housekeeper’s son, Robbie (James McAvoy). But that’s not how it looks to 12-year-old Briony. This overexcited young girl, who likes writing stories and plays, uses her imagination to turn McAvoy’s innocent Robbie into a sexual monster.

This is cleverly done. Briony's evidence hinges on a handful of adult intimacies that tell a quite different story when viewed through the eyes of a child. McEwan’s novels are adept at laying these traps. Knightley’s tawny beauty is both victim and provocateur, depending on how Wright replays the scenes. She and McAvoy are like the forbidden lovers in Brief Encounter: all starched passion and brittle cut-glass accents. McAvoy’s shy advances and impeccable manners betray his lower social standing, while Knightley is the playful aristocratic tease who tempts him to go further.

Working with Wright has really opened up Knightley's acting: the range of her performance here means that she has a tilt, at least, at an Oscar nomination. Poor McAvoy does a workable job of being one-dimensionally good - a burden for an actor of his range.

In Briony’s eyes, her glamorous older sister is tortured by McAvoy’s amorous attentions, but Briony’s sexual naivety is profoundly at odds with the erotic reality. It’s a great performance by the young Saoirse Ronan, here making her screen debut, and it is truly electrifying when she falsely accuses McAvoy’s Robbie of raping an underage friend. If only the rest of the film could sustain the tension.

The moment Robbie is jailed, the credibility of Wright’s film starts coming apart. The story takes a rude melodramatic knapsack off to war, and an older Briony (played by Romola Garai) signs up as a nurse and tries to atone for her ghastly sin.

The melodrama turns into a grim slog. Cecilia becomes a nurse on the home front; Robbie is freed from prison to go off to war. In this ranging section, after the tightness and brevity of the opening scenes, the film loses its direction.

Wright ambitiously mounts a long, continuous shot of British soldiers waiting to be shipped home from a French beach, with a Ferris wheel looming as a backdrop to scenes of chaotic carousing and horror. This is technically impressive, but overblown, and the final scenes set in the present day, as Briony reveals the substance of her atonement, are also flat and uninvolving.

After the promise of that luminous opening, the film meanders and ultimately loses its way. You may well leave " Atonement " feeling like Cecilia — mourning for what might have been.

Atonement is released in the UK on Sept 7

Inside the filming of Atonement

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010