Nanoka is a 15-year old high school girl who first encounters Koshiro at a train station when she accidentally drops her wallet. The two later encounter one another outside Koshiro's workplace and the two go on a date to an amusement park where Nanoka connects with him after both relate their woes with love, Nanoka dealing with unrequited love from a boy she had interest in while in junior high school. The two are shocked to later learn from their father they are long-lost siblings who had not seen one another in years since their parents' divorce. While seemingly accepting of having Koshiro in her life as her older brother, Nanoka does find herself often irritated with his cold and blunt personality coming from his internal conflict over how to treat her. Over time though, she comes to find herself slowly falling for Koshiro and being naive of the social norms regarding bonds between siblings.