The story takes place in the fictional town of Upper Darby,
Rhode Island in 2004. Anna Fitzgerald's older sister, Kate, suffers from
acute promyelocytic leukemia, a blood and bone marrow cancer. Anna was born as a
savior sister specifically so she could save Kate's life through the donation of her umbilical cord blood. At first it is successful, but the cancer continues to relapse throughout Kate's life.
Anna is usually willing to donate whatever Kate needs, but when she turns 13, she is told that she will have to donate one of her
kidneysdue to Kate's kidney failure. The surgery required for both Kate and Anna would be major; it is not guaranteed to work, as the stress of the operation may kill Kate anyway, and the loss of a kidney could have a serious impact on Anna's life, since Anna is fond of
hockey. Anna petitions for medical emancipation with the help of lawyer Campbell Alexander, so she will be able to make her own decisions regarding her medical treatment and the donation of her kidney.
At first Sara, their mother, believes that Anna's decision is led by a simple need of attention, but Anna is serious and refuses to withdraw her complaint, so the case has to be discussed in front of a judge. Having been a civil attorney before becoming a mother, Sara decides to represent herself. The judge assigned to the case is Judge De Salvo, who the year before lost his twelve-year-old daughter because of a drunk driver.
Recognizing that it is impossible for Anna and Sara to be together during the trial, Judge De Salvo hires Julia Romano, the court-appointed
guardian ad litem whose job it is to decide what would be best for Anna. Julia was once romantically involved with Campbell when they went to high school together, but Campbell broke her heart when he left her. Unbeknownst to Julia, Campbell left her because, after an accident that resulted in a
concussion, he developed
epilepsy and thought she deserved better.
Meanwhile, Anna's older brother, Jesse, who has spent most of his life being ignored in favor of ill Kate or donor Anna, spends most of his time setting fire to abandoned buildings with homemade explosives and using illegal drugs. He is a self-confessed
juvenile delinquent, which leads to their father Brian eventually confronting Jesse for his behavior.
During the trial, it is revealed that Anna is acting under her sister's wishes: Kate is tired of living; she's prepared to die and doesn't want to force Anna to donate a kidney that will likely not be adequate to save her life, so she encouraged Anna to gain the independence that has always been denied to her. As Anna stands up to testify, Campbell Alexander has an epileptic seizure and, thanks to this, Julia discovers the reason of their breakup, leading her to swear to Campbell that he doesn't need to hide his illness from her, and they can be together. The judge rules in Anna's favor, and grants Campbell a medical
power of attorney. After gaining medical emancipation, Anna hints that she plans to donate her kidney to Kate, wanting her sister to live. However, as Campbell drives her home after the trial, their car is t-boned by a truck. The on-call
firefighter, who happens to be Brian, Anna's father, arrives at the scene, retrieving an unconscious and severely injured Anna from the wreckage of the crushed car.
Anna goes into cardiac arrest and is revived, but when she and Campbell are rushed to the hospital, the doctor informs Sara and Brian that Anna is
brain-dead and asks them if they have considered organ donation. An injured Campbell steps in and declares that he, as the one who has the power of attorney, allows the donation and Anna's kidney is successfully transplanted. Kate survives the surgery but Sara and Brian are devastated: after so many years lived hoping to not see their first daughter die, the death of Anna is something completely unexpected and utterly unfair, but there's nothing anyone can do so the life-support machines are shut off.
After eight years, the Fitzgerald family is still struggling to cope: Jesse has reformed, graduated from the police academy, and has been awarded by the mayor for his work in uncovering drugs; Campbell Alexander and Julia Romano are married, but they don't often meet up with the Fitzgerald family, as the memory of Anna's death is too painful; both Sara and Brian needed years to accept their daughter's death, while Kate has recovered from her leukemia and has become a ballet teacher. She still feels deeply guilty, believing that Anna's death could have been avoided if she didn't push her to sue their parents, but feels relieved remembering that a part of Anna keeps living inside her through the kidney that, at the end, truly managed to save her life.
Tldr; parents create baby to save sister. Sister died because other sister sued for medical rights to her own body, despite being created to keep sick sister alive.