Alfonzo Cuarón’s “Children of Men,” loosely based on the 1992 P.D. James novel of the same name, is the type of big-minded film that could easily implode under its own heft: a far-reaching narrative about a dystopic future in which 18 years of worldwide infertility have left London as the last inhabitable city with an all-too-realistic resemblance to present-day Iraq.

But the support that Cuarón and cinematographer Emmanuel Lubezki provide transfuses unquestionable vigor into the movie’s plot about the imminent end of humanity. Handheld cameras, ambling, photographic asides and a cheeky, off-kilter humor retain a sense of hope and vitality – or at least amused, cynical resignation – in a futureless world.

“Children of Men” opens in a street-side café filled with morning commuters gathering around a television set for the announcement of “Baby Diego’s” death. Baby Diego is the world’s former youngest individual, at 18 years, who is killed by a fan after refusing to sign autographs. Among the commuters, Theo (Clive Owen), an employee for the Ministry of Energy, arrives for his morning coffee, lingers momentarily with the others, and leaves just before a bomb explodes. A woman emerges from the dust and debris, distraught and ravaged, carrying her severed arm. A sharp ringing spills into a straightforward title of white text on black: “CHILDREN OF MEN.”

The effect is jarring, immediate, and, like the unglamorous title shot, the film’s pretensions are offset by Cuarón’s direct, vérité treatment. He often allows Lubezki’s camera to linger over the soft, ashen grays and chromatic blues of the Orwellian London and its countryside: two murdered cops lay dead on a two lane highway in the hinterlands; a kitten claws at Theo’s leg; tall, narrow trees populate an otherwise barren forest – vestiges of indiscriminate death, prospective youth and endured time.

The attack is surrounded by speculation as to the responsible party. The media and government suggest a terrorist group called “The Fishes,” who operate behind an ostensible agenda of egalitarianism. Their primary platform is the freedom of all the refugees, or “fugees,” who have flocked to London as their countries have turned to ungovernable, nihilistic ruin. The Fishes, in turn, blame the government, claiming corruption.

The party, led by Luke (Chiwetel Ejiofor), kidnaps Theo at the request of Theo’s ex-wife Julian (Julianne Moore). She urges Theo to call on his cousin Nigel, a civil servant and patron of the arts, whose high-rise apartment is an unexpected utopia in which Michelangelo’s “David” (a bionic leg from the knee down) and Picasso’s “Guernica” seem to freeze time and offer a haven from the calamity below. Nigel’s privileged position provides the only opportunity for Julian, Theo and The Fishes to transport a young woman named Kee (Claire-Hope Ashitey) – a refugee and the first pregnant woman in 18 years – to “The Human Project,” a purported group of scientists dedicated to reversing infertility.

Each encounter introduces both hope and despair as confidences are made and betrayed, bringing into question the verifiability of the characters’ political and ideological alignments.

It is Theo’s longtime friend Jasper (Michael Caine), an aging, avowedly conspiratorial and hopeful bohemian, who supplies Theo with the assured benevolence that only comes with hope. He lives in a house silently tucked away in the woods.

His home is markedly less cluttered by the media that pervade nearly every corner, street and underpass of London proper, and the media’s overt presence in the film exposes our own contemporary obsession with information. The buildings and streets are war-torn, and cars are rare and relatively unimproved, but transparent screens appear throughout the city blaring messages of homeland security, patriotism and “Quietus,” a government-issued suicide kit that “Lets you decide when.” Only the media, it appears, has progressed.

“Children of Men” imagines life without youth in brash and unpolished, but nonetheless sublime, images. With the world’s sense of hope having expired without the rejuvenating spirit of youth that we celebrate today, the film allows us to consider how our present fascination with youth might exist in our lives, down to the very word “news” itself.

One of the film’s last images shows Theo and Kee adrift in a boat. A fog blankets the gently undulating water as planes soar overhead, dropping bombs on the city behind them. Despite the weight, Cuarón offers a last shred of hope: a dystopia behind them and open water before them.