buckle up because it's a lot of drama
Kathy H. is a student at Hailsham, a boarding school that emphasizes health and creativity. She attends with her best frenemy, Ruth and her friend Tommy, whom she has a bit of a crush on. As the years go by, the students learn from Miss Lucy, one of the Guardians, that their entire purpose in life is to be organ donors and then completing, a.k.a. dying. They will never become movie stars, or travel, or anything like that. They are clones of existing people who were created for the sake of organ harvesting. Ouch, I know. And an even bigger ouch, Ruth starts dating Tommy.
When their grade graduates at 16 years old, the trio is transferred to the Cottages where they stay until they're ready to become carers, the people who take care of the clones who are recovering from donating their organs. At the Cottages, they meet Chrissie and Rodney, a clone couple. They ask the trio about deferrals, a rumor circulating around Hailsham in which a couple can avoid donations for 3 years if they can prove tha they're in love. They also discuss the idea of "possibles", a.k.a. the people who they have been cloned from. They visit Norfolk to see if they had really witnesses Ruth's "possible", to see if the theory that a clone will end up in the same life as the original is true.
ex) if your original is a nurse, supposedly your clone would become a nurse eventually
Ruth's "possible" probably wasn't even her possible. It was just a vague resemblance
Afterwards, Kathy and Ruth go from frenemies to enemies when Ruth starts suspecting Kathy of hooking up with Tommy behind her back and starts being meaner to her than usual. This leads Kathy to leave the Cottages early and start her training to become a carer.
Almost 15 years later, Kathy is now a carer. And a good one at that. While most of her colleagues were fired? (the government told them that they can retire and become donors now) after 3-5 years, she has been going strong for 12 years. After discovering that Ruth is having trouble with her current carer, she takes over to hopefully find that the old, kind Ruth still resides in her somewhere. Thankfully it does. Slowly, the two become friends again. They decide to meet Tommy again at an abandoned boat. By this point, Ruth is close to death. She just completed her 2nd donation and she is close to completing. At the boat, she struggles with basic motor skills and is overall very sick. Tommy is doing great.
On their way home, Tommy and Kathy become close friends (kind of flirty friends) and Ruth kind of picks up on it. On her deathbed, Ruth encourages Kathy to date Tommy and be his carer. She completes, writhing in agony, but understanding that Kathy will take her advice and date Tommy.
Kathy and Tommy are having a fun time being in a relationship and all that jazz. However, there's fear lingering in the air. Tommy's fourth donation is rapidly approaching and they both know that he probably won't live to see another day. So the coupe decides to visit their retired Guardians to ask if the rumor about possibles and deferrals are true. They say that the rumor about deferrals was nothing but a dying wish, but that possibles were partially true. While there's no guarantee that you will end up like them (since you'll die of your donations by then), you are technically the same person so there is someone out there who is your "possible". The Guardians also say that there was emphasis on health and creativity because,
health: so that your organs are healthy for donation
creativity: a way of advocating for the humanity of clones. Their creativity showed that clones were human too. But their protest/advocation was shut down
After leaving, Tommy throws a tantrum knowing that he is now condemned to die. He and Kathy "break up" to protect Kathy since he doesn't want her to see him suffering. After the fourth donation, you'll be on life support so you'll just be an organ incubator so you don't have a carer or anything.
Upon leaving his side, Kathy lives on to care for other donors and lives her life knowing that she will one day become a donor and die too. It ends in a somber tone, as she acknowledges that both Ruth and Tommy have died, and accepts her fate as a clone.