On those rare occasions when we really admire a film, what we respond to is the magic by which the creator has swept us up into its world, transported us from our time to its time, taken us by the hand and immersed us, along with its characters, in its plot and mood and life and atmosphere. It's a rare achievement to have done it, particularly when so many films are cut-and-paste versions of some studio executive's vision of what will sell tickets. But rarely an artist will emerge whose work doesn't fit comfortably into the studio pattern, and who for some reason has been given carte-blanche to make his own film in his own way with his own cast and crew and with his own vision to guide him. And with the right artist and the right story what can emerge is a work of genius. That's how I feel about Alfonso Cuarón's "Children of Men." It's not as though he came out of nowhere; Cuarón is Mexican but he's made films in both English and Spanish; besides "Children of Men" he can claim "Y Tu Mamá También," "The Little Princess," "Great Expectations," and of course "Harry Potter and the Prisoner of Azkaban."

The film is based on P.D James's 1992 dystopian novel "The Children of Men," set in 2027, about a world in which the kind of horrors we see today in the streets of Baghdad have taken over the world. "Only England soldiers on," in the words of an exhortative slogan. But England and apparently most of the rest of the world are now fascist; immigrants are rounded up, sheltering one is punishable by death, death in fact is everywhere in the streets. Bombs explode without warning; rebels stone buses and trains. Worst of all no one is fertile; no child has been born in eighteen years, and no one knows why. A Department of Energy functionary in London, Theo (Clive Owen in the performance of his career, playing against his looks and his charm), who once was a rebel but now is a passive bureaucrat, is blindfolded and kidnapped one day, but not by the police; he's taken instead to meet his old lover, Julian (Julianne Moore), who asks his help to transport - well, a miracle - Kee, a young woman who is pregnant. This unlikely samaritan must take her to a ship waiting offshore, a ship that belongs to something called the Human Project, where she can safely have the baby. She needs transit papers, and Theo has a wealthy cousin Nigel (Danny Huston) who might help. Huston has been accumulating art treasures in his office - we see Michelangelo's 'David,' Picasso's 'Guernica' - and reluctantly agrees to get them.

Theo and Kee embark on a journey that may take them to the ship but is more likely to kill them. They take refuge in the isolated home of Theo's old mentor Jasper (Michael Caine), who now dispenses wisdom and marijuana in equal measures. They move on to another safe house which turns out not to be safe at all. And the closer they get to the sea the more brutal the world becomes. This is a film in which the atmosphere is as important as the action: The skies are a sooty, industrial grey; the sun seems not to shine. And against that darkening, grey world, the film takes us - along with Theo and Kee - through a maze of soldiers, rebels, and an anarchic kind of war, to a possible haven.

And then, as they approach the shore they find themselves in the middle of an astounding street battle between soldiers and rebels. (As an old director I always find myself fascinated by difficult scenes to stage, to choreograph, to design and shoot; this sequence in "Children of Men" is so astounding a piece of directing work, cinematography by Cuarón's favorite Emmanuel Lubezki, and editing by Cuarón, it will surely become a staple of film school curricula.) But Cuarón doesn't pause to admire his work; everything is in the service of his film and his story.

"Children of Men" is not easy to appreciate on first viewing; its images are so strong, its pace so furious that we respond viscerally, without thinking. It's only on reflection that the film's power and depth appear; this is the classic definition of a sleeper, and a sleeper that is destined to become a classic.