WHEN Hollywood producers sit around casting an action movie, it's hard to imagine any of them saying, "Hey, how about getting that skinny Scottish guy from Atonement?" Physically, James McAvoy is a most unlikely action hero, even after putting on some muscle to play one in Wanted.

To their credit, the film's producers (there are 13 of them) took that leap of faith and imagination, and the gamble pays off handsomely.

The key to its success is the foregrounding of McAvoy's Wesley Gibson as a meek, downtrodden accountant. His father left home when he was a week old. His boss gives him a hard time. His girlfriend is having an affair with his best friend. When he Googles himself he gets "no results". And Wes is a regular at pharmacies, stocking up on pills for his anxiety attacks.

It's in a pharmacy that Wes meets Fox (Angelina Jolie). She tells him his father was one of the greatest assassins who ever lived. "The man who killed him is behind you," she adds nonchalantly, cueing a manic shoot-out and chase through the streets of Chicago.

Fox is a member of the Fraternity, an organisation of assassins founded 1,100 years ago by weavers using a "loom of fate" - threading and binary codes - to pinpoint their targets. As weavers do.

Wesley wouldn't hurt a fly, but within minutes of meeting Fraternity leader Sloan (Morgan Freeman), he is ordered to shoot the wings off flies in a wastebasket. Sloan gets all poetical as he solemnly tells Wes that his father handled a gun like a symphony conductor. After a gruelling, violent initiation, Wes is assigned to take out Cross (Thomas Kretschmann), who, he is told, is the rogue member who betrayed the Fraternity and killed his father.

The script then becomes a frame on which to hang the relentless action as director Timur Bekmambetov orchestrates a succession of delirious sequences. He exhibits a fetish for bullets and following their trajectories, generally in slow-motion.

Wantedappears to be set in Chicago only because of the visual opportunities offered by the city's curving, elevated L train system, and it makes the most effective use of public transport in a thriller since Keanu Reeves boarded the bus in Speed. Another train figures in the movie's most elaborate set-piece, which is spectacular and outrageously funny. And Danny Elfman ratchets up the mayhem with a lush, robust score.