As the heartbreaking diagnosis of acute promyelocytic leukemia shakes the Fitzgerald family to its core, young Kate finds her world irrevocably changed. Her parents, firefighter Brian and lawyer Sara, are devastated to discover that neither they nor their eldest son Jesse has the necessary genetic compatibility to save Kate's life. Struggling with the weigh of despair, they make the desperate decision to conceive a "designer" savior sibling, Anna, through in vitro fertilization. From the moment Anna is welcomed into the world, her umbilical cord is harvested, and she begins an austere journey of donating compatible blood and stem cells to Kate, which entails a lifetime filled with hospital visits, intense treatments, and countless sacrifices.
As Kate's condition worsens, Sara clings firmly to her belief that Anna is a willing donor. In contrast, Brian grapples with the ethical dilemma of utilizing their younger daughter as a means to prolong Kate's life. The bond between Kate and Anna begins to weaken, especially as Kate battles guilt over how her illness has affected Jesse, who struggles silently with undiagnosed dyslexia.
Approaching her 15th birthday, Kate's health takes a terrible turn, leading to an agonizing decision for 11-year-old Anna: she must sacrifice one of her kidneys for her sister's survival. This decision weighs heavily, as it threatens Anna's own quality of life. Seeking independence, Anna consults attorney Campbell Alexander, who grapples with his own struggles of epilepsy and takes on her case without charge.
As Anna's legal fight for partial termination of parental rights unfolds, tensions rise, creating a rift between her and Sara. In an effort to shield Anna, Brian moves her into his fire station. Amidst this turmoil, Kate meets fellow cancer patient Taylor, and their fleeting romance brightens her spirits, but tragedy strikes when Taylor unexpectedly passes away. This loss drives Kate to a dark place, leading to a desperate moment that Anna courageously intervenes to prevent.
As the trial looms, Sara strives to protect Anna’s autonomy in court. In a moving gesture, Brian brings Kate home for one final day of freedom, granting the family a beautiful beach outing before the impending trial. In a moment of confrontation during the hearing, Jesse bravely takes a stand against Sara's intense questioning, encouraging Anna to share her truth. It’s revealed that although Anna showed willingness to be a donor, it was Kate who, understanding the life-threatening risks, implored her sister to refuse the surgery. This revelation creates a turning point in Sara’s perception, ultimately leading her to the heartbreaking acceptance that Kate’s time has come. Surrounded by her mother's love, Kate peacefully slips away in her sleep.
With Kate’s passing, the family begins to mend, bolstered by the news from Campbell that Anna has won her legal battle. In this newfound sense of closure, they start to navigate through their grief and forge ahead. After putting her career on hold, Sara returns to work with renewed vigor. Brian moves into a new chapter, leaving firefighting to dedicate his life to counseling at-risk youth, using his experiences to influence others positively. Meanwhile, Jesse looks ahead and enrolls in college, prepared to embrace the future.
As the years pass, Anna maintains a tradition of visiting Montana each anniversary of Kate’s birthday, a place that held special significance for her sister. Throughout these reflections, Anna learns that her existence was not solely meant to save Kate; rather, she was born because she had a sister, and their bond continues to flourish even after death.
