You write the reviews: Gone Baby Gone, Cinemas Nationwide

Crime and violence are such Hollywood staples that it's rare to find a film where the viewer feels the full shock of their impact. But then Gone Baby Gone is a far cry from the glamorous portrayal of an underworld that might be expected from the directorial debut of a big name such as Ben Affleck. Instead, it's a grim portrayal of petty lowlifes who, as the opening narration puts it, "started out in the cracks and then fell through".

This is a gripping detective story about the search for a missing four-year-old, and its theme is social divisions. The girl's trashy, drug-using mother, Helene, provokes moral outrage among most of the cops investigating the case, and it soon begins to look as though her shady connections and half-hearted approach to child-rearing might have played a part in her daughter's disappearance.

Amy Ryan is fantastic as Helene, alienating the audience with her brash unpleasantness before eliciting its sympathy with sudden flashes of humanity and grief, throwing its prejudices into uncomfortable relief. The large gap between these opposing perspectives is uneasily bridged by the central character, Patrick, a private investigator hired by Helene's sister-in-law in the hope that his background in the neighbourhood will open doors that are closed to the cops.

The director's younger brother, Casey, gives a compelling and nuanced performance as Patrick, whose girlfriend and partner, Angie (Michelle Monaghan), is perturbed at the way he ingratiates himself with the locals, many of whom he grew up with. His refusal to condemn a group of people written off by everyone else lies at the heart of the film, so the shocking dilemma that he faces at the end is only the natural climax of his struggle with his divided loyalties.

The rookie director creates real tension and a palpable sense of dread. There is no assurance that we can expect a happyending, and some scenes – such as a lakeside rendezvous with a neighbourhood crime lord and a nocturnal raid on a suspect's house – are among the scariest things this reviewer has seen for a long time.

The film's violent moments do not feel like showy action scenes but come across as genuinely horrible and disturbing, making the viewer's fears for the little girl's safety all the more intense. The complex story builds up to a series of twists that are firmly grounded in the film's plot and theme, making for an ending that has real emotional weight.

Priscilla McClay, TV listings producer, Leeds