Twenty-eight-year-old Koshiro Saeki, who lives with his father Zenzo, is dumped by his girlfriend Shoko of two years, who claims he was too cold and she had found someone else. While on a train the morning after, he sees high school student Nanoka Kohinata looking at a button and crying. Later, as she is getting off the train, she unknowingly drops her train pass and Koshiro follows to return it to her. Koshiro finds himself watching her sudden smile as she notices the
cherry blossoms are in bloom. Later, Koshiro is leaving his job at a marriage-arranging company with a coworker when he encounters the same girl again. Having two free tickets to a nearby amusement park, he gives them to her. To his surprise, she asks him to go with her. While on the
Ferris wheel, the girl explains that she was crying on the train because she'd been rejected by a boy she had been in love with for several years. Koshiro ends up telling her about his own break up, and cries while she comforts him. As they leave the park together, having enjoyed the outing and expecting to part and never meet again, they run into their father and are
shocked to discover they are siblings. Nanoka moved to Tokyo just that morning to live with them because their home is closer to her new high school. As Koshiro hadn't gone home the night before, he hadn't learned of her arrival. Since they had grown up living separately, they didn't know what the other looked like. As the series progresses, Koshiro finds himself unable to rid himself of the attraction that he felt for Nanoka when he first met her. Instead, his love and desire continue to grow, despite his attempts to fight them. Entering womanhood, Nanoka also develops feelings for her older brother, only increasing Koshiro's struggle. In near desperation, he moves out of the family home to remove himself from temptation and attempts to keep his coworker, Kaname Chidori, from finding out the truth behind his brusqueness with Nanoka. However, the solution is only temporary, as Nanoka begins visiting Koshiro regularly, cooking him meals and spending time with him. Eventually, unable to resist their feelings anymore, they share a passionate kiss and have sex with each other. Unsure of what to do now that they have broken a societal taboo, they visit their parents, before contemplating committing suicide together. In the end, they decide to live, and to continue their relationship. ==Characters==