Maddy, about to turn eighteen, lives in Los Angeles. She has spent her whole life indoors, due to a diagnosis of Severe Combined Immunodeficiency (SCID). Any contact with the outside world puts her at risk of a fatal infection or allergic reaction. The house is elaborately sealed, with an airlock at the front door. Maddy is cared for by her mother, who is a doctor, and by a full-time nurse, Carla. When Maddy was just a few months old, her father and brother were killed in an accident.

A new family moves in next door. The father drinks and is violent. Maddy and the boy in the family, Olly, become friends over the Internet. They also pantomime conversations through their bedroom windows. Eventually, Carla agrees to let Olly come over for in-person visits. Carla monitors the visits and does not tell Maddy’s mother about them. Soon, Maddy and Olly’s relationship takes a romantic turn.

After Maddy impulsively rushes out of the house to intervene in a fight between Olly and his father, her mother finds out about the visits. She fires Carla. In the fall, Olly starts school. Maddy stays in touch with him and tries to repair her damaged relationship with her mother. Her mother gives her a framed photo of their family, taken in Hawaii shortly before her father and brother died. One day, Maddy decides to risk her life in order to live fully. She books a short trip for her and Olly, to Hawaii.

After Olly gets over his misgivings, he and Maddy fly to Hawaii. They spend a wonderful, intimate day together, but in the middle of the night, Maddy wakes up with aches and fever chills. Olly calls an ambulance to take her to the hospital. Meanwhile, Maddy’s mother has learned where they were and has boarded a flight to Hawaii. When she arrives, she puts Maddy on a plane back home.

Maddy and her mother try to resume life as before. Maddy misses Olly but believes she must shut her heart to the possibility of love. She deletes all of Olly’s emails, unread. Her mother, seeing how sad Maddy is, rehires Carla. Olly and his mother and sister move out of their house, leaving his father behind, alone.

Maddy receives an email from the doctor who treated her in Hawaii. The doctor questions whether Maddy has SCID and urges her to consult a doctor other than her mother. Confronted, Maddy’s mother insists that Maddy really is sick. Searching her mother’s office at night, however, Maddy finds no record of a SCID diagnosis, nor any records of the years of treatment. A specialist contacted by Carla confirms: Maddy does not have SCID. The shock and grief Maddy’s mother felt over losing her husband and son expressed itself as a pathological need to protect Maddy from all possibility of harm.

Maddy is angry at her mother and wants to leave home, but Carla counsels her to be more compassionate. Maddy books a flight to New York. During the flight, she begins to forgive her mother. In New York, she and Olly happily reunite.