The story follows 18-year-old Madeline Whittier, who is being treated by her mother for severe combined immunodeficiency, 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. A family moves in next door and Maddy watches them from the window. Olly, the son, befriends her and the two begin to message each other online. She also notices that Olly's father is abusive and that his sister has a smoking problem. One day, Carla sneaks Olly into Maddy's house, and the two meet face to facefor the first time. They begin meeting in Maddy's house regularly, and at one point Maddy even goes outside for a few seconds. When her mother discovers Maddy has been secretly meeting Olly, she fires Carla and bans Maddy from ever seeing Olly again, but they continue secretly talking. One day, Pauline shows Maddy a photo of their family in Maui when Maddy was only months old. Influenced by this photo, Maddy decides to risk it all and go to Hawaii with Olly. Olly first disagrees, but Maddy lies— she tells him that she is on a new medicine that will keep her from getting sick— and Olly agrees. The two go to Hawaii, explore the country and are blissful for the day. The next day, Maddy has to be taken to a hospital because she wakes up extremely sick. Maddy's mother brings her home and hires Carla again to take care of her. Maddy eventually recovers. She stops emailing Olly as she doesn't want to miss him and the world. A month later Maddy sees Olly, his sister, and his mother, all loading their belongings into a moving van while their father is at work, escaping his tyranny. Two months after Hawaii, Maddy gets a letter from the doctor who treated her when she got sick there, and in the note, the doctor says she thinks that Maddy does not have SCID, and she got sick because she had spent her whole life inside and has never formed a natural immunity. The doctor blames myocarditis as the reason for Maddy's heart-stopping. Maddy, angry and panicked, searches her mother's medical files but does not find the test results or doctor's notes that would confirm she had SCID. Instead, she finds some notes her mother wrote and a few articles about SCID from the internet. She confronts her mother, who breaks down and indirectly admits she does not have SCID. Maddy tells Carla, and Carla says she had always suspected that her mom hadn't completely recovered from the death of her father and brother. Maddy's mother says that right after Maddy's father and brother died, Maddy got very sick, and her mother, not wanting to lose her, decided she had SCID, and needed to be kept away from the world. Maddy is mad at her mom and can't bring herself to forgive her. Maddy flies to New York, where she sent Olly on a mini scavenger hunt that led him to a used bookstore where she was waiting for him.
Reception
A reviewer for The Guardian wrote about the book: "The way the author describes Madeline's world using such beautiful imagery makes the reader appreciate the little things in life".
