Conceived by means of in vitro fertilization, Anna Fitzgerald (Abigail Breslin) was brought into the world to be a genetic match for her older sister, Kate (Sofia Vassilieva), who suffers from acute promyelocytic leukemia. Anna knows if she donates a kidney, she will be unable to live the life she wants; she will no longer be able to take part in extracurricular activities such as cheerleading and soccer, or be a mother. Her family members are introduced one by one and each tell about how Katie's illness has affected them personally and the family. When Kate turns 15, she goes into renal failure. Knowing that she will have to donate one of her kidneys to her sister, Anna sues her parents for medical emancipation and the rights to her own body. Attorney Campbell Alexander (Alec Baldwin) agrees to work for Anna as her guardian ad litem, suing for partial termination of parental rights. It is later learned he agreed to take the case not for the notoriety, but because he is an epileptic and understands her predicament of not having control over one's own body.
The judge refuses to grant summary dismissal, and the case goes to a hearing. The film is interlaced with flashbacks that detail the strong relationship between Kate and Anna as well as how Kate's illness has affected her siblings' lives and their relationships. Kate also meets a fellow cancer patient, Taylor (Thomas Dekker), who becomes her boyfriend both in and out of the hospital. He asks her to the hospital's "prom" for patients and they made love. A few days later, Kate is crying because Taylor hasn't called her for several days. Her mother Sara, furious because Kate said they "did it", storms out to ask the nurse where he is and learns that he has died. Before the hearing, their dad Brian takes the kids to the beach at Kate's request and with the doctor's permission. Overprotective as ever, Sara is infuriated when she finds out but Brian drives away anyway. Sara later joins them and they enjoy one last family outing together.
It turns out that Kate does not even want Anna's kidney, she has no confidence in another operation and prefers to die, and since Sara does not listen to her she has requested Anna to file for medical emancipation, and refuse to donate a kidney. Their older brother Jesse had known about it all along and urges Anna to tell but she doesn't dare say any more so he reveals it himself. Sara finally stops fighting Kate's decision, and Anna does not donate her kidney to Kate. Kate dies in her sleep at the hospital with her mother in her arms. After Kate's death, Campbell brings the court decision – Anna won the case. The family moves on with their lives, being changed by Kate's death, but every year on Kate's birthday they go to Montana, which was her "most favorite place in the world"