The movie starts out at the Hailsham boarding school, but you can see the peculiar ways the teachers have in keeping the children healthy. I soon found out that they were clones intended to provide vital organs for the non-clones.
Ruth (Pirates of the Caribbean & Pride & Prejudice's Keira Knightley), Tommy (The Social Network and the new Spiderman's Andrew Garfield) & Kathy (The Education, Bleak House & Northanger Abbey's Carey Mulligan) have a close friendship while there. Ruth is outgoing with her own strong opinions, while Kathy develops a liking for Tommy but with no romance involved. Ruth and Tommy pair-off like other adolescents while at Halisham.
As young adults the three move to the cottages (residential complexes), they begin their contact with the outside world. Kathy is often the peacemaker between Tommy and Ruth. Complications lead to Kathy becoming a carer - someone who takes care of other donors for a period of time before they themselves become a donor.
Tommy and Ruth become donors and Kathy ten years later is still a carer. Kathy visits a hospital where Ruth is on her second donation and it will probably be her last. Tommy is also on his final one and they all get together and find out Halisham has closed.
The three year deferral (for couples in love) they heard about proves to be false. Ruth dies shortly after and the movie ends after Tommy completes his final donation as Kathy now becomes a donor and also will eventually die.