The film begins with on-screen captions explaining that a medical breakthrough in 1952 has permitted the human lifespan to be extended beyond 100 years. It is narrated by 28-year-old Kathy H as she reminisces about her childhood at a boarding school called Hailsham, as well as her adult life after leaving the school. The first act of the film depicts the young Kathy, along with her friends Tommy and Ruth, spending their childhood at Hailsham in 1978. The students are encouraged to create artwork, and their best work gets into The Gallery run by a mysterious woman known only as Madame. One day, a new teacher, Miss Lucy, quietly informs the students of their fate: they are destined to be organ donors and will die, or “complete”, in their early adulthood. Shortly afterward she is fired by the headmistress, Miss Emily, for sharing this revelation with the children. As time passes, Kathy falls in love with Tommy, but Ruth and Tommy begin a relationship and stay together throughout the rest of their time at Hailsham.
In the second act, Kathy, Ruth and Tommy, now teenagers, are rehoused in cottages on a farm in 1985. They are permitted to leave the grounds on day trips, but are resigned to their eventual fate. At the farm, they meet former pupils of schools similar to theirs, and it is revealed that they are all clones. They also hear rumours of the possibility of “deferral” – a temporary reprieve from organ donation for donors who are in love and can prove it. Tommy becomes convinced that The Gallery at Hailsham was intended to look into their souls and that artwork sent to The Gallery will be able to confirm true love where it is present. The relationship between Tommy and Ruth becomes sexual, and jealousy causes Kathy and Ruth to break their friendship. The lonely Kathy leaves and becomes a “carer” – a clone who is given a temporary reprieve from donation as a reward for supporting and comforting donors as they are made to give up their organs. Tommy and Ruth’s relationship ends.