Ben Affleck's directorial debut was pulled from last year's London Film Festival because its subject, the hunt for a missing child, had very distant yet still distressing echoes of the Madeleine McCann case.

Gone Baby gone (15)****
Dir: Ben Affleck
With: Casey Affleck, Michelle Monaghan, Morgan Freeman, Ed Harris

Ben Affleck's directorial debut was pulled from last year's London Film Festival because its subject, the hunt for a missing child, had very distant yet still distressing echoes of the Madeleine McCann case. Although that inquiry tragically remains open, the film is now being given its British release. It's a remarkable drama, provocative for all the right reasons, and a movie that remains in the mind for a long time after viewing.

Casey Affleck, brother of Ben, plays Patrick Kenzie, a private eye asked to investigate the disappearance of a four-year-old girl. Affleck junior makes an unusual gumshoe, yet with his pinched features and slacker's slouch he is just the man for the job on the mean streets of Boston where Gone Baby Gone is set. This is the grim side of Beantown, where the accents are thick and the life prospects anorexic. Kenzie, as his voiceover tells us, specialises in missing persons, not the kind who disappear from normal lives, but those who "started in the cracks and fell through".

Amanda has been gone for several days by the time Kenzie is hired by her desperate aunt. By this point, Ben Affleck, who co-wrote the script with Aaron Stockard from Dennis Lehane's novel, has already shown the screenwriting ability that won him an Oscar for Good Will Hunting. He has established the story's edgy tone and Kenzie's unconventional character. Now he sketches the scene with swift, confident strokes. One of the cutest is a head and shoulders shot of a newsman in jacket and tie delivering an earnest piece to camera. Affleck then pans down to reveal the journo is wearing jogging shorts and training shoes. So much for being respectful. Frantic cops, rubbernecking crowds, the ravenous media out in force - it's a scene all too familiar from many a news bulletin, but Affleck is giving us an inside glimpse of the macabre circus.

It doesn't get any more reassuring when we meet the mother of the missing girl. A drunk and a drug addict, Helene (Oscar-nominated Amy Ryan) is a photofit of a feckless parent. Angie (Michelle Monaghan), Kenzie's partner in life as well as business, barely bothers to hide her contempt. It's a forgiving member of the audience who won't feel the same. Kenzie is different, however. Armed with his faith, represented by a cross and a St Christopher around his neck, and bolstered by the pleas of Amanda's God-fearing aunt, Kenzie doesn't have a job any more, he has a mission, and he'll walk through any shadowy valley in search of answers.

In doing so he must also do battle with Morgan Freeman's touched-by-tragedy police chief and Ed Harris's ferociously streetwise detective, who wisecracks that the boyish Kenzie should get back to reading Harry Potter. Between this lot, Angie, and Amanda's family, the tale plays out. With the story tearing along, it's at least an hour before you dare look away from the screen.

It might be thought that any director working from a detective novel has had half his job done before shooting begins. The best crime thrillers, after all, are masterclasses in slick storytelling. Affleck made a wise choice with Lehane. A previous novel of his, Mystic River, was turned into a double Oscar-winner by Clint Eastwood. But Affleck brings a lot of himself to the table. Among several inspired visual touches is a shot of Kenzie from behind, in shadow, his edges blurred to reflect the melting of moral boundaries as the case progresses. Though Affleck lets his grip of the material slacken after that first taut hour, the story soon snaps back into shape.

At the centre of it all is Casey Affleck. Impressive as he was in The Assassination of Jesse James by the Coward Robert Ford, for which he received an Oscar nomination, he surpasses himself here. Then, he stole the picture from Brad Pitt. Here, he's up against such heavyweights as Freeman and Harris - putting in one of his finest performances in years - and he matches them, slug for slug.

Slightly built and less conventionally handsome than his older brother, Casey Affleck nevertheless has a more impressive screen presence. No matter what else is happening he's always the most interesting thing in the frame. Kenzie, complex as he is, turns out to be one of the most easily understood characters.

He is the hero at the heart of a modern urban tragedy. It is through him that tough questions are asked about society's treatment of children and those who would do them harm.

"You gotta take a side," exhorts Ed Harris's detective as Kenzie begins to doubt his direction. The question is: how?

Gone Baby Gone keeps you guessing till the end about how things are going to turn out. Even then, Affleck is not quite done prodding the audience's conscience. Whatever else you take from this haunting film, you've got to take a side.