The novel centers on 18-year-old Madeline Whittier, who is being treated for severe combined immunodeficiency (SCID), also known as "bubble baby disease". Due to this, Madeline is kept inside her house in Los Angeles, where she lives with her mother, a doctor. The story follows 18-year-old Madeline Whittier a half Japanese, half African-American 18-year-old who is being treated by her doctor mother for severe combined immunodeficiency (SCID), and therefore is not allowed to leave her house or interact with anything that has not been "sanitized". Her world consists of her mother Pauline, her nurse Carla, and the books she finds comfort in; with her father and brother having died a long time ago in a car accident. Maddy's life changes when a family moves in next door. She watches them from the window and learns that the family includes a father, mother, daughter named Kara, and a son named Olly. Olly befriends Maddy, and the two begin to message each other online. Meanwhile, Olly's father is abusive and Kara has a smoking problem.