Gone Baby Gone, (15)

Vigils, rallies, flyers and posters in aid of a missing four-year-old girl. Pale, haunted relatives talking distractedly to TV cameras. An atmosphere sour with distress, bewilderment and suspicion. The setting is Boston rather than a holiday complex in Portugal, but there can be little wonder that the film studio pulled the drama Gone Baby Gone from its scheduled release in the UK last year – we had just had a whole summer dominated by something horribly similar on the news. Even the casting feels eerily synched: the abducted child looks quite a bit like Madeleine McCann.

That closeness to home should not overshadow the achievement of the film, remarkable for a director's debut and perhaps doubly so for that director to be Ben Affleck, who had spent the last four years or so being roasted on a spit by the media. Another man might have gone into hiding, but Affleck, who once won an Academy Award as a screenwriter, is clearly made of sturdier, and smarter, stuff.

Gone Baby Gone, adapted from a novel by Dennis Lehane, is a deeply troubling piece of work, braced with psychological intricacy and a moral dilemma at its centre that could scarcely be more topical. Lehane also wrote Mystic River, which Clint Eastwood directed for the screen, and it is no offence to that movie to say that Affleck has been even more acute in exploring Lehane's ideas of conscience, abuse and punishment.

Partly, its power derives from the unassuming way it goes about its business. At first, we seem to be watching a tough but generic police procedural. Affleck's younger brother, Casey, plays Patrick, a private investigator who's spent his life in the same dingy, blue-collar Boston neighbourhood. When the uncle and aunt of the missing four-year-old apply to him for help in the search, it's because they believe that people who know the 'hood are a better option than the police.

Patrick considers the case, and, despite the misgivings of his partner and girlfriend Angie (Michelle Monaghan), he persuades her they should help: "We can't do any harm, right?" We'll see about that. A Boston police chief (Morgan Freeman) offers a wintry welcome and assigns them to shadow a police detective (Ed Harris), who's even less genial.

A tougher obstacle than all of these, however, is the missing girl's mother, Helene McCready (Amy Ryan), a sullen, foul-mouthed slut who hangs around bars and does a lot of coke. Patrick knew her at school, so he tries to be on her side – everyone else thinks she's a disgrace to motherhood. The desperate, squalid circumstances of the McCready house, where Jerry Springer blares from the TV and a child's bedroom looks like a clearance sale at Oxfam ("Did they kidnap the furniture, too?"), are so thoroughly integrated into the movie's texture that "good local colour" hardly seems an adequate description.

The script (by Affleck and Aaron Stockard) follows suit, catching the profane, sarcastic backchat and hair-trigger aggression of working-class Boston: you wouldn't want to look the wrong way at anyone around these parts. One fantastic early scene has Patrick and Angie visit a bar where scuzziness seems to crawl off the walls; first the bartender ("Big Dave") snarls at them, then a local turns nasty and Patrick meets their hostility with some whipcrack violence of his own. You can almost feel the adrenalin buzzing through him.

That's another reason why the film delivers such a clout – it is quite brilliantly acted. Affleck, who caught the eye last year as Robert Ford opposite Brad Pitt's Jesse James, has a meek, boyish face that might have made him a perfect Smike in Nicholas Nickleby – how can this kid be a private eye, you wonder? Yet his slightly cracked voice and tense carriage make him interesting as well as sympathetic. He never overacts, even when Patrick is provoked in that bar-fight, and the thoughtfulness that hangs like a cloud over him becomes absolutely central to the story's accelerating moral uncertainty. When Angie says that she is "proud" of him in the aftermath of a brutal shoot-out, Patrick can't take the compliment. "I don't feel easy," he says, and nor do we.

Ryan has less screentime, but she stamps herself on the part of the mother indelibly. She's a woman with a pathological aversion to responsibility, yet she's a livewire and so blindly self-centred as to be almost awe-inspiring. By the end you feel that you know her, maybe too well.

For the first hour, the film has the gravity and self-belief of a classic, and the pace and edge of the dialogue work beautifully. Once the plot strikes out on a different, unexpected tangent, it loses some of its gritty credibility, and I heard myself actually groan at a point when melodrama lays its clammy paw on the action. Without giving away the details, the film serves up a twist of staggering improbability, and you fear that the film is about to unravel in a heap. Yet in the final minutes Ben Affleck somehow yanks it back into focus and saves it with a scene of haunting ambiguity.

What thrummed just beneath the surface of its first half was a terrible sense of dread – the dread of losing a child, and the dread of finding a child murdered, or (perhaps even worse) of never knowing what had happened to them. The film sacrifices some credibility but arrives at a more complicated and disconcerting place than we could have expected. It enables itself to restate the question posed at the beginning – what constitutes a fit parent? – and then to broaden it – how far is the law entitled to play God? This will be one of the movies of the year.