Kate Fitzgerald has acute promyelocytic leukemia. Conceived by in vitro fertilization, her younger sister Anna was brought into the world as a Savior sibling. She is a genetic match to her older sister and can donate compatible organs, blood, and tissue to help her. When Kate turns 15, she goes into renal failure and 11-year old Anna knows that she will be forced by her parents to donate one of her kidneys. She also realizes that she may not be able to live the life she wants to lead.