Kou Kitamura, the son of the owner of Kitamura Sports, grows up next to the Tsukishima family's batting center. The proximity of their businesses has forged a long‑standing friendship between the Kitamura and Tsukishima families, so the children often move between their homes as if they were relatives. Kou and his older sister Wakaba share a close bond, while Aoba, who is a natural pitcher with excellent form, feels jealous of the time Kou spends with her sister. Determined to match Aoba’s skill, Kou secretly trains hard in baseball, even though he publicly shows little interest in the sport.