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. Subsequently, the film is narrated by 28-year-old Kathy H (Carey Mulligan) as she reminisces about her childhood at aboarding school called Hailsham, as well as her adult life after leaving the school. The first act of the film depicts the young Kathy (Izzy Meikle-Small), along with her friends Tommy (Charlie Rowe) and Ruth (Ella Purnell), spending their childhood at Hailsham in the late 1970s. The school seems to be somewhat unusual. Students are encouraged to create artwork such as paintings and poetry instead of science and maths normal for school children, and their best work gets into "The Gallery." There is also a strong emphasis on "keeping yourselves healthy inside" especially when it comes to smoking. At one point, a new teacher, Miss Lucy (Sally Hawkins) quietly informs the students of their nature: they exist only as donor organs for transplants, and will die - or, rather, "complete" - in their early adulthood. The following day Miss Lucy is "no longer working at Hailsham." As time passes, Kathy and Tommy fall in love, but Tommy falls into a manipulative relationship with Ruth. Ruth and Tommy stay together throughout the rest of their time at Hailsham.
In the second act of the film, the three friends, now young adults, are rehoused in cottages on a farm. They are permitted to leave the grounds if they wish but are resigned to their eventual fate, seeing it as inevitable. At the farm, they meet former pupils of schools similar to theirs, two of which one day sight a woman in a nearby town who they believe to be a "possible" for Ruth, her "original" - the person she was cloned from. Ruth is ecstatic at the prospect, but when she, Kathy, Tommy and the two witnesses travel to the coast to re-examine the woman, there turns out to be very little resemblance. Ruth, bitter and disillusioned, rages that all donors are "modeled on trash," meaning that they are cloned from the people lowest in society, or, in her words, "in the gutter."
From the others, Kathy and her friends hear rumors of the possibility of "deferral" – a temporary reprieve from organ donation for donors who are in love and can somehow 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 verify true love. He hereby hints at his feelings for Kathy, but she misinterprets his words to signify that he wants to apply for a deferral with Ruth. She is visibly distressed. The relationship between Tommy and Ruth becomes sexual, putting a strain on Kathy's friendships with the two. Kathy, feeling the need to distance herself, leaves the cottages to become a "carer" – a clone who is given a temporary reprieve from donation to do the job of supporting and comforting donors as they give up their organs. Tommy and Ruth's relationship ends shortly before her departure, though it is not depicted but revealed through Kathy's narration.
In the third and final act of the film, ten years later, Kathy is working as a carer. She has watched many clones gradually "complete" as their organs are harvested. Kathy has not seen Ruth or Tommy since the cottages. While working as a carer, Kathy happens to meet Ruth again, who is frail and unwell after two donations. They find Tommy, who is also weakened, and the three of them drive to the sea as a short trip at Ruth's request. There, Ruth asks for their forgiveness for keeping them apart. She admits she has always known that Kathy and Tommy were meant to be together because their love for each other was real, whereas Ruth was with Tommy because she was jealous of his closeness to Kathy and afraid to be "left alone." She tells them it was the worst thing she ever did and now she wants to put it right, then claims she has found a means to do so: she has found the address of the gallery owner, Madame from Hailsham, whom she thinks may grant deferrals to couples in love. With some reluctance due to skepticism, Kathy accepts the opportunity. Shortly afterward, Ruth dies on the operating table when another organ is extracted.
A moment passes in which she looks at me as though I’ve just asked the head of MI5 if I can have a go on his computer. 'Definitely not answering that. No!’ she exclaims with a big, angry laugh.
At this point the film’s publicist – who has been sitting in the corridor listening in on our interview – approaches the big, round table where the actress and I are talking. He glances from me to her, her to me.
Eventually he backs away, arms out to the side and still facing us as if ready to rugby-tackle me to the ground should I attempt any more personal questions. The atmosphere in the room is terrible.
I think back to the things I've read about Knightley before meeting her: how she always used to keep herself to herself on film sets, how, according to Carey Mulligan, 'she reserves herself for the people she really cares about’ and how, according to Knightley herself, she 'never’ attends events 'as myself, Never No! I’d rather keep some protection up.’