Koshiro and Nanoka fall deeply in love, then discover they are the children of their divorced estranged parents... making them brother and sister. How will their relationship turn out?
Koshiro remembers the birth of his sister and their life together before their parent's divorce. Although they were too far apart in age to understand each other, they felt a vague sense of affection. In the present, Koshiro goes to see his mother for the first time in many years. She tells him how much Nanoka looked up to her big brother and tells him to be a good big brother to her. On the ride back, he remembers an incident when baby Nanoka gave him a glass of mud and he ended up keeping it because it made her so happy. He resolves to be a good big brother to her.
