"Koi Kaze" is a 13-episode anime series that explores complex themes of love, family, and societal boundaries. The narrative centers on Koshiro Saeki, a 27-year-old man working at a wedding planning company, who is struggling to move on from a recent breakup. His life takes an unexpected turn when he encounters a 15-year-old high school girl, Nanoka Kohinata, on a train platform. The two feel an inexplicable connection, though neither understands why at first. Later, Koshiro discovers that Nanoka is his younger sister, whom he had not seen since their parents divorced and she moved away with their mother years earlier.

As the story progresses, Koshiro and Nanoka begin to rebuild their sibling relationship, but their feelings for each other grow increasingly complicated. Koshiro wrestles with guilt and confusion over his growing romantic attraction to Nanoka, while Nanoka, unaware of their blood relation, develops her own feelings for him. The series delves into their internal struggles, portraying the emotional turmoil and moral dilemmas they face as they navigate their forbidden bond.

The supporting characters, including Koshiro's colleague Chidori Kaname and Nanoka's friend Koyuki, provide additional perspectives on the situation, though the focus remains firmly on the central relationship. The narrative avoids sensationalism, instead opting for a grounded and introspective approach that examines the psychological and emotional dimensions of the characters' experiences. The series concludes with an open-ended resolution, leaving the future of Koshiro and Nanoka's relationship ambiguous but emphasizing the weight of their choices and the societal constraints they confront.