Other than joint-custody of a title, the character names, a few of the set pieces and one lengthy monologue, there's little that Alfonso Cuarón's film Children of Men shares with P.D. James' novel, which is to Cuarón's advantage.

James' strained Christian parable of a future when men and women become infertile and children slowly vanish from the world and memory is certainly more meditative than this action-packed dystopian film. But it is also too dependent on chance encounters to drive the plot, while its message of Christly redemption becomes heavy-handed, down to a risible Bethlehemite finale. Cuarón's film is just as hopeful as James' book, but without the tablespoons of theology, becoming something far more universal and humane.

The world — or the part that remains habitable after ecological and political disasters — has sunk into grimness. It's been 18 years since the birth of a child, and people have all but abandoned dreams, as the future has become blatantly finite. The streets of smog-socked London are piled with rubbish and debris, while the ever-present eye of a fascistic authority keeps watch (the year 2022 of Soylent Green and the 2027 of Children of Men form a continuum).

As he trudges to work one morning, Theo is kidnapped by a group that the government has labeled as terrorists: the Fish. The leader of the gang, Julian (Julianne Moore), is Theo's former wife, whom he hasn't seen in years. Now Julian needs Theo's help in getting some documents for a woman that the Fish are trying to smuggle out of Britain to a safe haven in the Azores.

What follows is a harrowing journey that will inevitably shake Theo from his moral torpor, as a handful of friends and strangers unite to save the life of a single woman who may very well be the only hope for humanity's continuation.

Children of Men is a wrenching experience. Cuarón and his writers imbue the bleak, dying landscape of England with images that hadn't yet been conjured when James wrote her novel, particularly the fruits of America's moral mission at Abu Ghraib. The phrase "homeland security," utilized throughout the film, seems enough in this disintegrating culture to excuse any amount of horror. James created a quasi-fascist state in her novel that now seems almost quaintly Orwellian (complete with distraught women pushing prams with dressed-up kittens as surrogate children). Cuarón's 2027 posits Baghdad not as a historical incident, but a prophecy for the entire world.

Inhabiting this blasted set is a host of superb actors. Clive Owen's intricate performance as Theo is precision acting, taking a droop-shouldered husk of a man and transforming him into a heroic protagonist. Michael Caine's Jasper is equally affecting, and his finest work since 2002's The Quiet American. Unfortunately, there isn't enough of Moore, but her scenes with Owen are powerful.

Chiwetel Ejiofor as the rebel Fish leader, the excellent Pam Ferris as a long-unemployed midwife, and the great Romanian actress Oana Pellea as a tough refugee give Children of Men one of the strongest film ensembles this year. Claire-Hope Ashitey, as the young woman Kee for whom everyone risks their lives, is radiant, spicing her moving performance with some rather wicked humor.