It’s hard to single out one moment in director Alfonso Cuarón’s Children of Men that doesn’t feel urgent, tense or, more frighteningly, very plausible. Set in a dystopian, war ravaged future Britain where no child has been born for 18 years, the film effectively draws a straight line between the bleak, bombed-out future on the screen and current events, including the war in Iraq and the crackdown on illegal immigration.

“This is where we’re heading,” Cuarón is trying to say, but Children of Men, based on the book by P.D. James, is remarkable not because of its social commentary, but for the way it doles the commentary out.

Cuarón — clearly a follower of the “show, don’t tell” philosophy — has crafted a thrilling technical achievement here, creating a gray, violent British police state full of visual exposition that shows us everything we need to know. We’re not told the world is in chaos, we see it in the newspaper clippings with wartime headlines, the downbeat TV news reports about casualty counts and terrorist bombings, the political graffiti and in the faces of caged refugees on the street.

Corporate structures and bureaucratic buildings are all that stand intact - it’s hard times for sure. It’s anyone’s guess as to who started the war or who’s on the right side, but that information isn’t really pertinent to the film’s simplistic story.

Clive Owen cuts a decidedly glum figure as Theo, a former revolutionary turned bureaucratic drone who now kills time by getting high with an aging pot dealer (played warmly by Michael Caine). Theo’s revolutionary past comes back to tap him for a favor in the form of his ex-wife (Julian Moore), the leader of a political extremist group. Moore’s faction is protecting a young woman (Claire Hope Ashitey) who, somehow, is pregnant. It’s up to Theo to use his political pull to get the pregnant woman, named Kee, to another political group called The Human Project. There, supposedly, Kee will get the medical treatment and care she and her baby will need to survive.

Children of Men is a surprisingly economic film of ideas, but the film’s driving story is too simplistic. While the world around them is burning with conflict, the film’s main characters follow a standard road movie formula. There are a few shocking scenes along the way, but if you’ve seen the trailer, you know where the film’s heading.

Children of Men is most effective when the turbulent world outside comes crashing in on the main characters, leading them into chaotic and shockingly realistic scenes of guerilla warfare and terrorist action. There’s a tense, stunningly shot stretch in the second half of the film in which Owen runs through an urban war zone, evading sniper bullets while those around him fall bloody to the ground. The scene rivals anything I’ve seen in recent war films, including the opening shots of Saving Private Ryan.

There’s much to recommend here, including good performances by the cast (especially Hope Ashitey), great music and profound cinematography. I only wish the main story would have taken a few more unexpected turns. Maybe then the film would have held the gripping pace it sometimes achieves.