Sheltered since early childhood, Katie Price lives with a life-threatening sensitivity to sunlight caused by the rare genetic condition, xeroderma pigmentosum (XP). She is housebound during daylight hours and is cared for by her father (Jack) and her best friend (Morgan). Katie comes out of the house every night, once the sun has set. One night, her longtime crush, Charlie, notices her while playing guitar at the train station. Katie leaves suddenly and forgets her notebook, which Charlie keeps. He returns the next day and gives it to Katie when she shows up to retrieve it. He explains to her how he got his injury that prevented him from getting a scholarship to the University of California, Berkeley before kissing her.
However, Katie has yet to tell Charlie of her condition, despite her father warning her to do so. Charlie takes Katie out for a night to Seattle, where they go to a live show, and Charlie makes Katie play one of her songs on one of the city’s streets. Once home, they go swimming out in the lake, and dry off with a fire on the beach. Charlie mentions watching the sunrise, and Katie runs home in fear. Charlie picks her up and quickly drives her there, but she does not make it in time, and is exposed to the sunlight for just a couple of seconds. Katie runs inside, while Morgan and Jack come home soon after. Charlie is still standing confused at the front door, and Morgan explains Katie’s condition to him. Once the doctors run some tests, they come to the conclusion that Katie’s brain is contracting, and it’s only a matter of time before she dies.
Katie begins to experience twitches in her finger, which prevents her from playing the guitar. She also ignores Charlie’s messages as she does not want to hurt him. Jack eventually convinces Katie to speak to Charlie, who still wants to be with Katie, and does not care about her condition. Katie goes to Charlie’s swim meet with the Berkeley coach, and they hang out at the house with Morgan and Jack. Charlie takes Katie out one night, and surprises her by booking a recording session, where she sings a song she wrote for him. Soon after, while hanging out at her house, Charlie mentions that he has to visit the boat for the last time, which he has been hired to take care of the whole summer. Katie, fearing that she is going to die, eventually remembers the time Charlie told her that he wished they could sail together, and convinces Jack to let her go with Charlie, despite it being during the day. Katie sails with Charlie, feels the sunlight, and spends her final moments with him, dying shortly thereafter.
Some time later, Charlie goes to Katie’s house where he bids farewell to Jack as he is going to pursue his swimming dreams. Jack tells Charlie that Katie wanted him to keep the notebook. Later on, Charlie is setting out, where he listens to Katie’s song on the radio and reads the heartfelt message which Katie wrote for him in the notebook. In the letter, Katie tells Charlie to watch for the new things which are coming his way and to look up in the sky and to always remember that she loves him.