Kō Kitamura, son of the owner of Kitamura Sports, lives in the same neighborhood as the batting center run by the Tsukishima family. Due to their proximity and the relationship between their businesses, the Kitamura and Tsukishima families have been close for many years, with their children going back and forth between the two homes like extended family. Because Kō and Wakaba are the same age and always together, Aoba is jealous of all the time Kō spends with her older sister. Aoba is a natural pitcher with excellent form, and Kō secretly trains to become as good as she is, even while publicly showing little interest in baseball. Then Wakaba dies in a swimming accident at a summer camp during fifth grade.