Madeline Whittier is a teenage girl with a type of Severe Combined Immunodeficiency, or "bubble baby disease". She lives in a house that is always sterilized and her room is completely white and always decontaminated. The only thing she can do is talk to her nurse (Carla) and mother, take online classes or read books, every day, week, month, and year. Madeline is seventeen years old, and she has never left her house in those seventeen years...
...Until she saw that Olly's family had moved in next door... "I look out my window, and I see him. He's tall, lean, and wearing all black - black T-shirt, black jeans, and a black knit cap that covers his hair completely. He catches me looking and stares at me. I stare right back. His name is Olly. Maybe we can't predict the future, but we can predict some things. For example, I, Maddy, am certainly going to fall in love with Olly. It's almost certainly going to be a disaster".
Madeline starts to email Olly, and he always replies back. They do this for many, many days, and become good friends. Madeline's window is also facing Olly's bedroom window, so they can try to mime their feelings and thoughts to each other. They do this until Carla (her nurse) schedules secret meetings, in which Olly and Maddy can see each other, and even touch each other briefly.
One day, Maddy decided that staying at her house the rest of her life was a lot worse than getting to see the world, so she asks Olly to go to Maui in Hawaii together with her. To convince Olly to go with her, she told him she had some pills to keep her condition under control... but there were none.
After Olly and Madeline have fun together on the beach and in the ocean, they fall asleep, but it turns out that Maddy had gotten very sick overnight and started dying. Her disease was activated somehow and it made her heart stop, but she was rushed to the hospital in time to be saved by adrenaline shots.
I need an ambulance!
During the time Madeline was getting well, Olly, his mother, and his sister moved away to New York, and they were separated. Maddy found out she did not have SCID, and decided to reunite with Olly. Maddy promised Olly that, if he found one of her favorite books in a bookstore, then he could have her back. And he did.