It focuses on the fact that instead of harvesting organs from the dead, people have been specifically "grown" I guess, only to have their organs removed in their 20s.
We watch Mulligan, Knightley and Andrew Garland grow up at a special private boarding school run by Charlotte Rampling. While there they are prepared for their life as donors. As they grow up, they discover the inevitability of their position, and try to talk their way out of it to greater or lesser success.
I enjoyed the film, but it was depressing. I wanted to yell at them "Run, fight do something," and got angry when they didn't. To a degree it reminded me of slave holding America, without the happy ending, and that irritated me too.
In short, you'll probably enjoy it, if you don't mind being depressed.