Koshiro is recovering from breaking up with his girlfriend. On his way to work in the train, he first meets Nanoka while returning a name tag that she dropped as she was trying to exit the train. After returning to his house from work, he finds out from his father that his sister, whose face he can no longer remember, will be moving into their house for easy of communting to school. His girlfriend visits him during lunch break the next day to get back the key to her house. At the end of the work day, a customer gives Koshiro and his boss two tickets to the theme park next door. Upon leaving their work place, Koshiro meets Nanoka again and spends the rest of the day at theme park with her; they consol each other on their love lives. Their father appears at the end of the day to pick Nanoka up, revealing to both that they are siblings.

突然一緒に住むことになった妹、七夏の「お兄ちゃん」攻撃に耕四郎タジタジ

クマちゃん交換したりちょっといい感じぃ♪がっ！七夏の様子が変で、耕四郎ドキドキ

元カノに七夏と一緒のとこを目撃され、弁明のため再会！久方の デートに耕四郎ソワソワ

男には知られたくないゴミがある。親切心で片付ける七夏に怒るに怒れず、耕四郎ヒクヒク

大事な妹に虫がついた！？ 猛烈に勘違いをしてドツボにハマり、 兄妹喧嘩、耕四郎ワナワナ

七夏がもらったラブレター、気が気じゃないのに無理をして大人な対応。耕四郎トホトホ

兄として距離をおく事でどことなく自分に好意を抱いてくれるようになった七夏に耕四郎ルンルン

思いが本物である事に気づくとともに 七夏と離れることを決意し、レッツ夜逃げ　耕四郎コソコソ

まっとうに社会人として生きようと決心したものの、逢いたい気持ちは募るばかり。耕四郎メソメソ

七夏との関係を千鳥に知られ、夢のような生活に危機！千鳥の自分を思っての叱咤に、耕四郎ボコボコ

道を外し、失った物の悲しみは小さくはない。が、はじめての疑いのない気持ちに耕四郎ワクワク

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.