Here's a very brief, spoilerific summary: Kathy, Ruth, and Tommy were all students at Hailsham, a boarding school where children are raised to eventually be harvested for their organs. It's set at some indeterminate time in the future, although there's nothing particularly futuristic about the actual story.
One interesting thing to me was the way the story was constructed. Throughout, there's never really any huge reveal. Ishiguro drops hints about the childrens' ultimate fate so that nothing that happens in the book really blindsides the reader (with the possible exception of the very end). In most novels, this would seriously undercut the dramatic tension, but in Never Let Me Go, it serves to ratchet it up, making the book more of a tense journey to an inevitable, foretold destination than a thrill ride to an unexpected conclusion. Because of this, my focus throughout the book was on the characters and the way they interacted with one another and I found it to be the most thought-provking aspect of the novel.
Unlike many books of this nature where the protagonists find out about an awful fate awaiting them and then fight it until their last breath, the characters in NLMG seem to, for the most part, accept their eventual fate. Until they are harvested, the subjects are caretakers of other donors, so they know exactly what will eventually happen to them, yet they've been indoctrinated throughout their lives that this is their ultimate destiny. Even Tommy and Kathy, who attempt to find a loophole, are blandly accepting once their last chance is finally shot down. It's just an interesting facet of human nature (and one that Christopher may have mentioned in his review) that people tend to adjust to just about any circumstances, given enough time.
The other thing worth noting is that the ending of the book is very affecting, maybe moreso than anything else I've read this year.