"Very odd, what happens in a world without children's voices."

It's 2027, and women can no longer make babies. Bummer, right? With Children of Men, Alfonso Cuaron and his fellow screenwriters, shockingly, don't bother to take examine the positive effects something like that might bring. Instead, Cuaron brings us an almost overbearingly dark and depressing tale about heartbreak, loss and terrorism.

As the film opens, the death of Deigo Ricardo, the youngest man on the planet (18 years old) has sent shockwaves through London. As most of the population breaks out the hankies, Theodore (a wonderfully low-key Clive Owen) drifts through the mourning crowds at a coffee shop with barely a glance at the TV screen. Even when the shop suddenly explodes after he leaves, all he can express later is annoyance that his ears are still ringing. It's an obvious but still effective way of showing that the people of Britain have become used to the chaos erupting around them.

To its credit, Children of Men never feels like the fantasy film it undeniably is. This is partly thanks to the well-measured and believable world the film creates (having lived here for years, it always surprises me how well London photographs as an apocalyptic wasteland); but it's largely down to Owen, who sells every scene and lends the film a much needed dose of humanity.

Owen is what really makes Men work. His Theo is a refreshing hero, in that he's not really a 'hero' at all. He has no real love interest, isn't all that likeable, and at times seems annoyingly ignorant and narrow-minded. But like all great heroes, when the time comes he always does the right thing. Theo's revolutionary past is re-awakened upon meeting Kee (Claire-Hope Ashitey) who, against all the odds, is pregnant. He joins forces with freedom fighter ex-wife Julian (Julianne Moore) and her colleagues, but events soon conspire to send him and Kee on the run with only each other to rely on.

The cast includes a vast array of British vets and newbies (Michael Caine, Chiwetel Ejiofer, Charlie Hunnam, Peter Mullan) none of whom fail to impress. Ejiofer especially continues to prove himself as one of the most talented breakout stars of the last five years. His presence is commanding and his final line heartbreaking. It's a travesty that he hasn't had a decent leading role since Dirty Pretty Things.

As Men proceeds, it only gets more gripping. Despite its flaws it truly sucked me in - as a thriller you'll find little better among the 2006 offerings thus far. Initially the film's complete lack of hesitance at killing off major characters in shocking eruptions of violence made it more engaging than it really had any right to be. After a while this shock tactic actually became a little tiresome (although still effective) but Men remains engaging to the end mostly due to the endearing character at its centre, and the adept handling of major dramatic sequences. Cuaron and cinematographer Emmabuel Lubezki (the man behind The New World) stage several sequences with a powerful usage of one take shots. Whether we're following Theo through a war torn building under seige by the army or watching him deliver a baby, the work behind it is astonishing on reflection.

Still, as you may have guessed, Children of Men is no five star film. The death, despair and depression soon reaches almost farcical levels. It's simply too much. Most of the characters have depressing backstories which, whether explicitly stated or only hinted at, collectively give every scene a kind of morbid lonliness which becomes overbearing. Only Owen carries off his dark past without becoming stereotypical at all. But I should note (again) that the performances are very accomplished. The great talent involved is simply let down by slightly lazy screenwriting. Here's hoping that Cuaron's next effort will be just as entertaining, but not quite as by-the-numbers.