The story unfolds around a young girl named Katie Price, who navigates life in a quaint house with her father in Washington State. Unfortunately, Katie suffers from Xeroderma Pigmentosum, a rare condition that requires her to avoid sunlight to protect her delicate skin. Local children, unaware of her condition, mockingly label her a vampire due to her reclusive nature. Adding to the narrative is a boy named Charlie who skates by, catching Katie's admiration—a crush she harbors in silence.
As the plot develops, one of the local kids knocks on Katie's door, but her father, Rob Riggle, denies her access to outdoor playtime, explaining that it's daytime. This leads to the introduction of Morgan, a determined girl who insists on seeing Katie and eventually becomes her closest friend.
Fast forward to present day, and we find Katie, now a teenager portrayed by Bella Thorne, graduating from high school after being homeschooled. Her father gifts her a guitar that belonged to her late mother, a heartfelt token from her past. As the sun sets, Katie begs her father to let her venture to the local train station to play her guitar, where, under the cover of darkness, she shines just like any other young person her age.
It’s at the train station that Charlie, played by Patrick Schwarzenegger, first hears her beautiful voice and is captivated. In a flurry of nerves, Katie hastily packs her belongings and flees, neglecting to take her journal with her. The next night, Morgan arranges for Katie to collect her journal from the station, but instead, she finds a charming surprise as Charlie himself returns it, igniting their blossoming relationship.
Days pass, and Katie invites Charlie to a party at Morgan's workplace. However, the initial gathering is disappointingly dull, prompting Charlie to whisk them away to an exciting party. Amidst youthful revelry and flirtation, it’s clear that Charlie's heart belongs to Katie as they share their first dance, leading to a sweet first kiss on a boat—an ephemeral moment that marks the start of their special connection.
With Katie still keeping her condition a secret, she tells Charlie, "I'm busy during the days, but can come out at night," and their romance flourishes under the stars. Charlie takes Katie on amazing adventures, including a surprise train ride to Seattle, where they witness a concert and enjoy each other's company under the enchanting night sky. During a beach excursion, Charlie proclaims, >“Get ready for the most beautiful sunrise.” However, Katie’s joy is shadowed by the reality of her condition as she frantically realizes the sun is about to rise.
In a frantic scramble, she manages to escape the sun’s rays but not without consequence. As the story progresses, Katie faces the harsh truth of her health, receiving advice from her doctor that her exposure to sunlight is causing significant danger to her well-being. She distances herself from Charlie at a critical moment, who is concurrently focused on his swim meet aimed at securing a college scholarship. However, motivated by Katie's encouragement to persevere, he dives deeply into both his sport and their relationship.
While Katie's father welcomes Charlie into their home for a heartfelt reconciliation, Katie reveals her vulnerability, recounting how she’s loved him from her window for years. As Charlie fosters Katie's musical dreams by arranging a recording session where she performs “Charlie's Song,” her health begins to decline dramatically. Determined to embrace her final wish, she asks her father to allow her to sail with Charlie despite the glaring sunlight.
In a bittersweet moment, they create lasting memories sailing through flashbacks of their romance. Eventually, the story concludes with a poignant scene where Charlie, Morgan, and Katie's father scatter Katie’s ashes in the very waters where she shared moments with her mother.
The narrative encapsulates Charlie’s future as he embarks on his college journey at Berkeley, only to hear the echoes of “Charlie's Song”—a tribute to their enduring connection that has captured millions online. The heartfelt conclusion wraps up with Charlie reflecting on Katie’s final words of love and gratitude from her journal, showcasing the profound impact she had on his life.
