Due to their twelve-year age gap and their parents' divorce and relocation to different parts of Japan, Koshiro and Nanoka grew up with minimal knowledge of each other. However, their story begins when fifteen-year-old Nanoka moves to Tokyo to live with her father and twenty-seven-year-old brother, Koshiro, who works in a matrimonial company.
On their morning train commute, despite their age difference, Koshiro retrieves Nanoka's railway pass and they feel a temporary connection before separating. Destiny, however, brings them back together again near an amusement park, where they spontaneously go on a date and have an open conversation about their romantic problems.
During this date, Koshiro breaks down and Nanoka comforts him. It is then that Koshiro realizes his genuine attraction to her. However, their joy is short-lived as they discover that they are actually siblings upon meeting their father.
Koi Kaze delves into Koshiro's struggle to abide by societal standards, as he battles his feelings for Nanoka. Additionally, the story portrays Nanoka's growth into womanhood while falling in love with her brother.
