Twelve years apart in age, siblings Koshiro and Nanoka were separated when their parents divorced and relocated to different parts of Japan; consequently, they grew up with barely any awareness of each other's existence. The narrative starts when 15-year-old Nanoka moves to Tokyo to reside with her father and brother, who is 28 years old and works for a marriage-arranging company. Koshiro first meets Nanoka on a train one morning, returning her dropped train pass. Despite the age gap, they experience a fleeting spark of connection before parting ways. Shortly thereafter, they cross paths again near an amusement park, where they embark on an impromptu date. While on the Ferris wheel, the pair open up to one another, discussing their recent romantic troubles. Nanoka comforts Koshiro as he breaks down in tears, and he realizes he is strongly attracted to her. Immediately following the date, they meet their father and are shocked to discover they are siblings. Koi Kaze centers on Koshiro's love for Nanoka, his internal conflict between obeying societal norms and his feelings, and Nanoka's maturation into womanhood alongside her developing feelings for her brother.