Gritty, depressing and utterly relentless, 'Gone Baby Gone' showcases not only the abilities of Ben Affleck as a director, but also the growth of his younger brother Casey, who excels in his biggest role to date.

Based on the crime thriller of the same name written by Dennis Lehane and based in Boston like his 'Mystic River', the two brothers use their knowledge of the area to bring a healthy dose of realism to the story.

When two young private detectives, Patrick Kenzie (Casey Affleck) and Angie Gennaro (Michelle Monaghan) are hired to take a closer look at the mysterious disappearance of a little girl they soon find themselves caught up in a moral maelstrom as they uncover a world of filth and gross negligence.

Armed with a litany of sharp one-liners and a knowledge of the streets, Kenzie works his way through the underbelly of working class Boston, but while the one-liners serve to bring some light-hearted moments, they never bring a smile. The plot is simply too off-putting for that?

Paedophiles, dirty cops, crackheads and murderers as well as pitiful parents dominate the landscape as Kenzie and Gennaro search in vain for missing four-year old Amy McReady, discovering that the truth here is not so simple as a missing child. Nothing is as it seems.

The investigation is further disrupted by a pair of cops (Ed Harris and John Ashton) who aren't necessarily interested in helping out as well as Amy's mother (a superb Amy Ryan), who seems more inclined to chase down her next fix than find her little girl.

It all leads to a messy end as the truth is slowly uncovered, leaving Kenzie with a choice: between his own morals or what he is legally obligated to do. And, like everything else in this dark, depressing story, the decision is not as simple as it seems, and regardless of his final choice the consequences are dire.

'Gone Baby Gone' rescues the older Affleck from the career nosedive he has been in since 'Daredevil', while announcing his younger brother as a serious leading man. But be warned it is not easy viewing and will leave you unsettled long after the credits have rolled?