In suburban Los Angeles, 18-year-old Maddy Whittier is being treated for the immune disorder SCID by her mother, Pauline. Her nurse, Carla, has helped take care of Maddy for 15 years. Pauline constantly monitors her daughter’s health status and provides daily medication. Only she, Carla, and Carla’s daughter, Rosa, are allowed in the home. Maddy isn’t allowed to leave the house or interact with anyone or anything that hasn’t been “sanitized.” She yearns to see the world, especially the ocean.

One day, a new family moves next door, and their son, who is Maddy’s age, catches her eye. Later that night, while she and Pauline are watching a movie, the boy and his sister appear on their doorstep, offering a bundt cake. Pauline politely rejects it, and as she is closing the door, the boy asks where her daughter is. She lies, telling him Maddy isn’t home. As Pauline sees Maddy’s desire to get to know the boy, she tries to block off all opportunities for her to contact him. It’s also revealed that his father is violent and their relationship is strained.

Later, the boy writes his number on his window for Maddy and soon they begin communicating through text. He introduces himself as Olly, and they text for a while, getting to know each other and eventually growing very fond of each other. Knowing her mother wouldn’t approve, Maddy convinces Carla to secretly let Olly visit her inside the house, though Carla makes them promise to stay on different sides of the room from each other. She later invites Olly over on the Fourth of July, as her mother is working that day. Maddy and Olly share a passionate kiss as fireworks go off outside.

The next day, Maddy sees Olly fighting with his father outside. When his father shoves him to the ground, she, to Pauline’s shock, rushes outside to comfort Olly. Deducing that Maddy and Olly have been seeing each other behind her back, she forbids their relationship and later fires Carla for betraying her trust. Maddy decides it’s time she took matters into her own hands, and using a credit card she had previously opened online, she buys tickets to Hawaii, and convinces Olly to go there with her. On the way to the airport, Olly calls his sister, Kayra, telling her that he’s going to Hawaii with Maddy for a couple of days and to take care of their mother.

In Hawaii, they share a romantic and life-changing experience. Pauline sends a police car to find Maddy, and when she spots Kayra walking by the house, she asks if she knows where Maddy and her brother are but denies it. During the trip, Maddy passes out and Olly rushes her to the hospital and she wakes up back in bed at home. She breaks off contact with Olly as she doesn’t want to make another mistake over love again; therefore, she’s unable to say goodbye when his mother finally decides to leave his father, taking Olly and his sister back to New York with her.

A doctor from the hospital in Hawaii calls Maddy to give her an update, telling her that she doesn’t have something as severe as SCID. Scouring her mother’s records, she can’t find anything indicating she’d ever been diagnosed with the disorder. Upset that her mother has been lying to her all her life, Maddy runs away from home and stays with Carla and Rosa. A doctor confirms Maddy has never had SCID, just an underdeveloped immune system from under-exposure due to spending most of her life in filtered air. Pauline later tells her that after her father and brother died in a car crash, she was all she had left and she wanted to keep her safe. Giving Maddy a bag of her things, she leaves.

Later, Maddy reunites with Olly in New York, where they restart their romance.