You probably are aware of this. It is set in the future, and begins with the death of the world's youngest person, an 18-year-old Argentinean. For unspecified reasons people are no longer able to have children, and as the human race stares at extinction it turns nasty. The first part of the film sets up the world – one of pollution, miserableness, social dislocation, and the like. Illegal immigrants are being rounded up and sent to resettlement camps while billboards advertise euthanasia pills ("Quietus - you choose the time"). Everyone has pets, people's computers are covered in cuddly toys, and bombs keep exploding (planted either by the pro-immigrant Fishes or by the government as distraction, it is never established).
Then the plot kicks in. The protagonist (Clive Owen) is contacted by his ex-wife (Julianne Moore), now a leader of The Fishes. They want him to use his government contacts to help them transport a teenage illegal immigrant girl to the coast, for initially unspecified reasons. After that things start happening at such a rate that it would give too much away to say anything else. I nevertheless cannot resist talking about the last section of the film, set in the Bexhill Resettlement Camp during an uprising by the illegal immigrants resettled there. You will probably never see a depiction of modern urban warfare as gripping, and watching this made me feel like I was seeing Jenin, Beit Hannoun, or Fallujah.
I am up against deadline pressure, and as always I am having trouble expressing myself, but if you just absorb that this is one of the best films I have ever seen then you get the idea. This film is so emotionally involving that I spent the last portion of the film terrified that the protagonists would cop a stray or targeted bullet. It is a sufficiently bleak film that a down-beat "Oh dear, they’re all dead" ending would have fitted, so my fear for the characters was maybe justified, but I do not normally feel so for two dimensional projections of people on a cinema screen. The other great thing about the film was the way it genuinely makes you think about a lot of things, and not just ones related to the central premise. And it was good that it never got into explaining the sudden collapse in human fertility - unlike with V For Vendetta, say, there was no "Blimey it was the government what did it!" moment.
I should also mention that a lot of the last part of the film is set in the countryside around Rye, so if you have ever been to All Tomorrow's Parties it will look somewhat familiar.