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.
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