Word of warning, don't watch the trailer past 1m20s if you haven't seen the film as it contains a pretty major spoiler; it was the only official trailer I could find, and I couldn't be bothered sifting through hours of fan made trailers to see if they contained spoilers...
In the near future, humankind has become infertile and the world has descended into chaos as a result. Theo (Clive Owen) is approached by his ex-girlfriend (Julianne Moore), now a fugitive, labeled a terrorist by the British government, the only government yet to have fallen, to smuggle a young woman out of the country. With Britain violently militarized and concentration camps set up for immigrants; Theo is forced out of his comfortable life to confront the harsh realities of utter desperation.
Not only is this one of the greatest concepts to a sci-fi film to have been made, but the cinematography is incredible. Cuaron uses his trademark long takes to make the viewer his or her own character taking in a lot of the world that Theo doesn't notice or remark upon. All of the characters of well developed and multi-layered; especially Theo, who, though essentially a fish out of water, is weary and experienced which makes him a part of the world; not somebody for the audience to imprint themselves upon.
Clive Owen, of course, gives a phenomenal performance in this role; Julianne Moore and Chiwetel Ejiofor are both fantastic as well. The only downside to this film is Michael Caine, who sticks out like a sore thumb, with an annoying character that seems to be an exposition tool and voices the philosophical themes to the audience. Without Caine, this would be in my top 10.