Readers notice early on in the novel that the narrative point of view switches between characters, and the timeline is not static. Although it primarily stays in the present, it skips to the past frequently. There is only one chapter, the epilogue, that occurs in the future.
The plot opens by introducing Anna and Kate as sisters. Kate was diagnosed with an aggressive form of leukemia in 1990 and was not responding to treatment or chemotherapy, sparking the need for her to find a suitable donor.
We then learn about Kate’s diagnosis as a baby. The story explores the fact that Anna was a “test tube baby,” conceived to fill the need for a donor to keep her sister alive. Anna is custom-made to be a perfect match for Kate’s medical needs, and from the moment Anna is born, she is pricked, prodded, and cut open in order to donate bone marrow and healthy cells to her sister.
The story then fast forwards 14 years through Kate’s illness and the life of the family. Through these years, we learn about Kate’s unending suffering, as well as the struggles that Sara and Brian face as a couple as a result of their daughter’s illness. We also get the impression that Sara is solely focused on Kate’s struggles and illness, taking attention away from her other children, Jesse and Anna (who is frequently undergoing painful and invasive procedures because of Kate).
The story takes a turn when 13-year-old Anna approaches attorney Campbell Alexander, asking him to represent her while she sues her parents for medical emancipation. Kate is in the final stages of kidney failure, and Anna does not want to give Kate her kidney. Alexander agrees to represent Anna for free.
When Sara is served with papers notifying her Anna is suing, she becomes outraged and does not understand Anna’s actions at all. However, Brian understands to an extent.
Kate becomes very ill and is hospitalized, given a week to live. The trial begins, and Sara, previously a lawyer, represents herself and Brian. During the trial, Anna admits that she filed the lawsuit because Kate asked her to, wanting to end her own suffering. Anna also admits that she does wish to have more personal freedom and cannot have that while being a donor.
After hearing testimonies from Kate and Anna’s doctors as well as Anna herself, the judge grants Anna medical emancipation, and Campbell is given medical power of attorney over her.
After the trial, on the way to the hospital to visit Kate, Campbell and Anna get into a near-fatal car accident. Anna suffers from injuries that have left her with irreversible brain damage. Campbell, who now has power of attorney, directs doctors to take her kidney and give it to Kate.
The final chapter of the book is the epilogue, narrated by Kate, years later. She discusses the extreme grief her family experienced after Anna’s death, but explains how Anna saved her, putting her in a lasting remission and how Anna will always be with her.