Anna Fitzgerald's older sister, Kate, suffers from acute promyelocytic leukemia, a cancer of the blood and bone marrow. Anna was conceived in order to harvest blood from her umbilical cord in order to use in treatments to help save Kate's life. Although the treatment was initially successful, Kate relapsed; ever since, Anna, the only compatible family member, has been used as a donor for any other bodily substance needed to treat Kate, who continues to swing between remission and relapse as she grows up.
Anna is usually willing to donate whatever Kate needs, but when she is 13, she is told that she will have to donate one of her kidneys. The surgery required for both Kate and Anna would be major; it is not guaranteed to work, as the stress of the operation may well kill Kate anyway; and the loss of a kidney could have a serious impact on Anna's life. Anna petitions for medical emancipation with the help of lawyer Campbell Alexander, so that she will be able to make her own decisions regarding her medical treatment and the donation of her kidney.
Anna's mother, Sara, is an ex-lawyer and decides to represent herself and her husband in the lawsuit. Over the course of the novel, she tries on several occasions to make Anna drop the lawsuit. Anna refuses to do so, but the resulting tension between her and her mother result in her moving out of the house to live with her father Brian in the fire station where he works. This is done on the advice of 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 school together, but Campbell broke her heart when he left her. It is eventually revealed he left her because he discovered he had epilepsy and thought she deserved better. They get back together at the end of the book.
Meanwhile, Anna's 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 home-made explosives, using the knowledge that his fire-fighter father gave him to make the fires bigger and fiercer, and doing drugs. He is a self-confessed juvenile delinquent and pyromaniac, and the only time throughout the book that his parents pay him any attention is when Brian discovers that it is Jesse who has been setting the fires. Brian forgives him, and by the end of the book, he has reformed and graduated from the police academy.
During the trial, it is revealed that Kate asked Anna to sue for emancipation because she did not want Anna to have to transplant, and because she believes that she will die anyway. The judge rules in Anna's favor, and grants Campbell medical power of attorney. However, as Campbell drives her home after the trial, their car is hit by an oncoming truck. Brian retrieves Anna, who is unconscious, and suffers an injury to the head, and Campbell, who suffers an injury to the arm, from the wreckage of the crushed car and rushes them to hospital. However, after some time, the doctor informs them that Anna is brain-dead, that the machines keeping her alive may as well be switched off, and asks them if they have considered organ donation. Campbell steps in, and declares that he has the power of attorney, and "there is a girl upstairs who needs that kidney." Kate is prepped for surgery, and Anna's kidney is successfully transplanted. Kate survives the surgery and goes into remission. Most of Anna's usable organs are removed for transplants in the future. Campbell and Julia get married, Kate stays in remission for at least 6 years, while Anna dies for her sister's life.