Fifth-grader Ko Kitamura was born on the same day in the same hospital as his neighbor, Wakaba Tsukishima, who treats him as her boyfriend as a result. One day during summer vacation, she makes him take her to swimming lessons on his bicycle. On the way back, to avoid classmate Osamu Akaishi, who has a crush on Wakaba, Ko joins a sandlot baseball game organized by another classmate, Daiki Nakanishi. Because he has never played before, Ko fields badly, but because he has practiced in the Tsukishima family's batting cages he hits the game-winning home run. After their joint 11th birthday, Wakaba gives Ko a schedule of birthday presents for him to give her through age 20. Soon after this, Wakaba drowns while saving a younger girl at an overnight swimming camp. Ko does not know how to respond to this until, the evening after her funeral, he sees Akaishi weeping outside Wakaba's house and realizes he just needs to cry.
The closing credits show time passing and by the end, Ko and Wakaba's younger sister Aoba Tsukishima are in junior high school.
樹多村光と、月島四姉妹は幼なじみ。しっかりものの長女、一葉。明るく素直な次女、若葉。ちょっぴり無愛想な三女、青葉。元気いっぱいの四女、紅葉。そんな夏のある日。若葉は、光と夏祭りに行く約束をして、キャンプに出かける。数日後、光が何気なくつけた高校野球に流れてくるニュース。光は耳を疑うが…。
스포츠용품점의 아들, 키타무라 코우는 배팅 센터 겸 찻집인 '클로버'의 츠키시마 자매와 소꿉친구로 야무진 첫째 이치요, 밝고 솔직한 둘째 와카바, 조금 무뚝뚝한 셋째 아오바, 기운 넘치는 넷째 모미지와 사이좋게 지내고 있었다. 코우는 와카바에게 부탁받고 수영 교실까지 자전거를 태워준다. 학교 제일의 미소녀이자 인기 많은 와카바. 그런 와카바와 사이 좋은 코우는 주위 동급생들의 부러움을 한 몸에 받는다. 그리고 그 중에는 학년에서 가장 싸움을 잘하는 아카이시도 있었는데...
