The book is better.

IT's easy for a good movie to color subsequent readings of the book it's based on. Literature is a subtle, fragile medium, more complex than cinema but also easily overwhelmed by what gets summoned up on screen. It's hard to re-read even so powerful a story as J. R. R. Tolkien's Lord of the Rings without Peter Jackson's Middle-earth crowding into the mind's eye, shunting the imagination aside. Pick up Raymond Chandler's The Big Sleep, and you're likely to picture Humphrey Bogart filling Philip Marlowe's suit; re-read The Prime of Miss Jean Brodie, and you'll probably hear Maggie Smith's brogue trilling the title character's every line of dialogue.

Sometimes, though, it works the other way: A great book can spoil a good film adaptation, by setting the bar so high that mere artistic success is insufficient, and the viewer spends the film regretting the places where the novelist's genius outshines the director's vision. That's the case with Children of Men, Alfonso Cuaron's adaptation of P. D. James's dystopian novel--it's an impressive movie, touched by flashes of inspiration, but it's best seen, and judged, by an audience that hasn't yet read the book.

Like James's novel, the movie takes place in a near-future world in which the entire population has been rendered infertile, and humanity is a couple of decades deep into its final four score and ten. The setting is Great Britain, the last outpost of order in a globe gone to bedlam. Or quasi-order, at least: There's a fascistic government bent on rounding up illegal immigrants, advertisements for suicide pills on television, and bands of flagellants on the streets of London. Our guide through this chaos is a brooding Clive Owen, lending his usual bitter charm to the role of Theo Faron, once a political activist and now a hard-drinking functionary in the Ministry of Energy. He slouches unmoved through a decaying landscape that's part 1990s Sarajevo and part Blade Runner, and the camera prowls around him--curious and appalled where he has gone numb, lingering over scenes of misery (bombs, sobbing women, rock-throwing vagrants) that he allows to wash off his back.

This routinized despair is interrupted soon enough, of course, when Owen's Theo is kidnapped--by his ex-wife, it turns out, now the head of a radical group known as the Fishes, who are bent on overthrowing the government, saving the immigrants from their miserable refugee camps, and restoring equality before the law. Or something like that: Theo's ex, played by Julianne Moore, is clearly on the side of the angels, but the rest of her merry band offer a cautionary lesson in the perils of idealism run amok. This is a movie with a tedious anti-Bush message woven in: The government's immigrant roundups take place under the aegis of "Homeland Security," and there are obvious nods to Fallujah, Guantanamo, and Abu Ghraib. But Cuaron is savvy enough to complicate his story's politics, to paint the rebels in shades of gray and even black, and to choose a "plague on both your houses" approach to his dystopia over an easy left-wing Manichaeism.

Besides, Children of Men isn't really interested in scoring political points--it's a chase movie, with global infertility as the MacGuffin that sets in motion Theo's flight across England to the sea, as the guardian of a young black immigrant who is mysteriously, miraculously, pregnant. Their goal is to escape England on a ship from the Human Project, a semi-mythical group of do-gooders with whom the Fishes are in contact. It's a journey that requires a harrowing descent into a refugee-camp hell, where government troops fight chanting Muslims under an eldritch sky, and then a rowboat ride into a foggy, oceanic unknown. Along the way, the child is born, to be carried bawling through a world of blood and ash, toward an uncertain salvation--for itself, and all the world.

The whole thing is bravura filmmaking--there are extended takes that will leave you breathless; gripping, layered performances from Owen and Michael Caine (sporting long hair and a pot-fueled twinkle as Theo's hippie friend and mentor); and strong supporting turns from Moore and the ever-excellent Chiwetel Ejiofor, who would be on his way to stardom if anyone could pronounce his name. (Clare-Hope Ashitey doesn't really register as Kee, the pregnant girl, but the script doesn't offer her much to work with: She's an icon of hope, a foul-mouthed Mary figure, not an actual human being.) And it marks a major step forward for Cuaron, whose best movie to date had been Harry Potter and the Prisoner of Azkaban, where his flair for cinematic composition was tethered to J. K. Rowling's imagination and the expectations of millions of fans. Otherwise, he has gravitated toward stories that are lovely but ultimately vacuous--like Y tu mama tambien, which was subtitled and foreign and fooled critics into overpraising it, or Great Expectations, which was in English, starred Ethan Hawke, and received the pans that it deserved. With Children of Men, though, he's put his talents in the service of a vision that's original, memorable, and moving, and whose themes of political despair and procreative failure should touch a nerve in anyone concerned about Western civilization's current prospects.

So long, that is, as the "anyone" in question hasn't read the book. If they have, they'll feel like a playgoer promised Hamlet and presented instead with an above-average murder mystery about a troubled son hunting down his father's killer: It's good so far as it goes, but it's a shadow of something greater. James's novel, which has the same outline as the movie but radically different details, makes Cuaron's film look cautious and conventional by comparison--its spiritual themes shallow and PC, its characterizations thin and predictable, its attempts at political complexity naive. This is a testament to literature's advantages, in a way, but it's also a sign of the perils of trying too hard to make something great your own. You can pilfer selectively from your source material and fashion something gripping out of it, but you're better off if nobody knows what you left behind. Which is why those coming fresh to Children of Men are better off seeing Cuaron's adaptation first, and only then turning to the greater pleasures, and challenges, of the story that the movie decided not to tell.