At the very beginning of the book, we learn about our narrator, Kathy, who serves as a "carer" for "donors." Throughout the story, we learn that these donations are actually of people's organs, and Kathy is responsible for calming these donors and helping them settle into the donation process.
Kathy then describes the school of Hailsham, a privileged school which required students to demonstrate their creative abilities through art donations. This art supposedly reveals the truth about each student's personality traits, therefore distinguishing how they will perform as donors.
At Hailsham, Kathy has two best friends, Ruth and Tommy. Ruth is constantly putting herself out there, asserting herself as a sort of leader. Tommy, on the other hand, is out there for the wrong reason. Tommy is constantly bullied due to his lack of creativity, and he would often have uncontrollable outbursts because of it.
After Hailsham, the students move to the cottages, which serve as a sort of college-like transition phase into donations. Each of the students here begin to demonstrate more maturity, and some even express new personality traits altogether. For instance, Ruth becomes more demanding of her friends in an attempt to gain popularity, but Kathy still sticks by her since they share valuable bedside chats where they can discuss life's worries with the comfort in knowing these discussions are private.
One day, the group decide to visit Norfolk in order to find a "possible" of Ruth, which is a person who lives the perfect life a student desires. These possibles perhaps serve as a source of hope for the students in some way, and upon realizing the possible is nothing like Ruth, Ruth ends up feeling absolutely shattered, and she begins to act even more aggressively towards Tommy and Kathy.
After Ruth ends up attempting to ruin the relationship between Kathy and Tommy, a wedge is driven between Kathy and her friends, which then causes her to sign onto becoming a carer before either of her friends.