What is the general plot line of Never Let Me Go?

This excellent novel
opens in Hailsham, a school where three friends, Kathy, Ruth and Tommy are introduced to us.
Their lives at school are explored, with many curious aspects to them that we do not understand,
such as the revulsion that some characters who visit the school show to the boys and girls.
Kathy likes Tommy a great deal, but at they grow up, it is...