Alan Moore and P.D. James could not be more different in terms of, well, almost everything. Although they are both excellent writers (one specialises in comic-novels and the other, words without the pictures), Moore leans towards the wild man, multi-ring fingered iconoclast angle, while James (a venerable literary lady) has an OBE and is about as representative of the great British establishment as the changing of the guard. She still writes under the initials P.D. and it's tempting to think that Phyllis Dorothy had to succumb to a publisher's request just as Joanna Kathleen Rowling did – boys don't read books written by girls, do they? Sheesh. So where's the connection?

At roughly the same time, both had imagined a very bleak British future (Moore's was published between 1982 and 1988, James' in 1992) and both have had that future fully and soberingly realised on screen. V For Vendetta, a movie lacking Moore's credit (all other Moore adaptations have been fiercely criticised by the hirsute author), still works as a deft political fantasy, a savage signpost indicating where we may be heading but the anti-government protagonist, V, is presented as almost super-human, an ideal, an idea. In Children of Men (and I cannot work out if the title is supposed to be ironic, sneering, straightforward, what?!), its ‘V' is a resistance movement throughout the British Isles that the fascistic government continually paint as terrorists. It's an extraordinary film, technically speaking, but it's also immensely entertaining in the bleakest way imaginable. Did I mention it was bleak?

It's the near future in an all too recognisable UK. Yes, the buses have plasma real time video advertising on their sides (an aspect of production design that reminds us that talented Mexican director Alfonso Cuarón's last full feature - thereby not including Paris, Je T'aime – was Harry Potter and the Prisoner of Azkaban). The dull blue-grey palette of the cinematography is (like The Matrix's green hue) almost too depressing to endure. Mankind is on its way out because women cannot conceive and the overwhelming blanket of doom has cast a pall on the world that few are able to shake off. Theo (a terrifically convincing Clive Owen) is a drone who walks out of a café after hearing that the youngest person on the planet (he was 18) has just been stabbed to death by a fan that was refused an autograph. Seconds later, the café is destroyed by an explosive – even this event doesn't shake Theo out of a deep-rooted melancholy. He 'feels like shit' all the time and works at a mindless job. We learn that his son's death has levered his life into a depressive pit and that he was once a political activist (with Julianne Moore as his partner, now one of the most feared terrorists of the UK underground). Suffice to say, the media do a lot of the shading and we are left to unravel this world with the zones of zeitgeist info available to us now. Very little cues an audience into when we are but what elements do, do enough. Theo's friend is political cartoonist Jasper, whose work looks extraordinarily like the Guardian's Steve Bell. He is played by Michael Caine and here, he is playful and funny and the political conscience of the movie. Caine's way out from underneath the crippling sadness is mostly marijuana based. And the actor has fun with Jasper – and we do too.

Owen is kidnapped, taken off the street like an Iraqi man with a wealthy wife. Scared (who wouldn't be?), he is taken to meet his old flame and mother to his dead child. As a father can I just say that to type the words 'dead child' makes one pause. She needs Theo to pull strings via friends in high places (oddly but in keeping with the movie's tone, this is Battersea power station complete with Pink Floyd's inflatable pig suspended outside. Why? Why not)? Theo needs to obtain a visa to allow a very special person down to the coast where she would meet up with a boat – onboard is rumoured to be "The Human Project," an offshore scientific concern that represents mankind's only hope for a future. Owen complies, gets the visa and has to set off with his special cargo as per the visa's purview.

The journey south starts off delightfully. Owen and Moore play the estranged couple to the hilt and a scene in a car involving ping pong balls is an utter delight – but then it's designed to be given what comes next. Here, Children Of Men doesn't so much change gear as startle explosively. Cuarón's skill is found in the almost perfunctory way the action and drama unfolds. He really does succeed in presenting the events as if they are really happening a la cinema verité. There is some insane stunt work or some artful CG as whole takes compress the most extraordinary physical events. This sequence is merely a stylistic taster for something special that's coming down the line.

INSERT: Anyone remember seeing an album/CD cover with a naked couple sitting on the front in a sort of psychedelic background? I think it may have been the score for a movie called Zabriskie Point. It remains unlistened to (Pink Floyd did the score, does that remark make me un-cool?). The point I'm making is that there are some movies out there that set up shop in your head and create a micro-mythology – purely because you are continually exposed to its supplementary material and the movie's content and merit are almost always a mystery until you make the effort to see the damn thing. A director I worked with recently made the intriguing remark that Children Of Men had one key scene that was extraordinary. That set the micro-self-placed-mythology in place. Talk about great word of mouth.

At the end of this key sequence, Chiwetel Ejiofor as Luke, makes a decision which plunges everyone in the car in so deep there is no turning back, literally in this case. Ejiofor is fast becoming one of my favourite British actors; he has great presence, an intriguing face and a comfortable screen quality that allows him to play almost anything (romantic comedy, deadly assassin or grubby political activist to name just three). I would imagine most professional actors would love a career of great variety. 'Chewy', as he's known, is doing just that. Back at a safe house, Theo finds out what all the fuss is about. His partner on the trip is a young pregnant woman. The resistance want to stop the government getting hold of her and use her and her baby as political leverage. Theo, whom Kee trusts absolutely because of what Julianne Moore's Taylor has revealed about him, only cares that she find safety, a place to have the child. Where do they go? A prison where immigrants are tortured of course, Abu Grahib style. It's the only way to get to the water and any hope that lies on the waves.

There are some very smart twists and turns that culminate in a sequence of extraordinary reality. Cueing this up is the birth of the child and for the first time in a mainstream picture that I can recall, we see a baby born relatively clearly. Whether the sleight of cinematic hand is an animatronic slipped from a stagehand under the bed or if it possibly could be ultra-sophisticated CG, the effect is intimate and moving – and that, my friends, is all that counts. Cuarón knows how the use the tools to serve his story and is not in thrall to the almost limitless possibilities from the tools available. And now we come to that sequence. I will not give anything away except to say that verité style, we stay with Theo and Kee throughout and as a battle erupts between the warders and the resistance (we're talking tanks, automatic fire, explosions, mangled and ripped up bodies) it feels real.

It's only afterwards you suddenly have a little but powerful epiphany. It was all one take… OK, I know that CG can stitch takes together and certain wall wipes can be artfully used to change the shot BUT it's the overall cumulative effect that is important. You feel every bit of the struggle to stay alive, you feel each and every close bullet ricochet and you can almost feel the heat of the explosions. It's masterful film-making made to look as if they just happened to be there when all this carnage erupted and that takes some skill. I must give a nod of appreciation to the army of craftspeople who made this shot(!) so seamless.

Yes, it's bleak. Yes, there is little evidence that the human race is worth saving (I mean we are pretty awful animals when push comes to shove). Yes, it's all shot in that depressing haze of hopelessness. But Owen, Caine and Moore all shine brightly and the narrative is so deftly handled by Cuarón that it all becomes somewhat thrilling once the ingredients are effortlessly conjoined. Add to this, moments of technical brilliance and you have a story that is well worth anyone's time.