Katsuya Niimi, a former junior high track star holding national records in the 100-meter and 200-meter sprints, enters Seishū High School and witnesses the baseball team's defeat alongside Yuri Nakao's tears in the stands. This moment spurs him and friend Susumu Karasawa to join the struggling team, initially aiming to make Yuri smile. He discovers Yuri is the coach's daughter and that the baseball program risks disbandment without success.

Though initially motivated by romance, Niimi cultivates a true passion for baseball. As center fielder wearing jersey number 8, his elite speed enhances outfield defense and base-running. His athletic transition capitalizes on sprinting prowess but requires mastering baseball skills and teamwork dynamics, evolving from an individualist into a unified team member who values collective effort among nine players.

Niimi’s romantic focus on Yuri Nakao defines his journey, their relationship complicated by external rivals like Kentarō Yamanaka—a star pitcher from a rival school and Yuri’s childhood friend. This rivalry fuels both diamond confrontations and romantic tension. Simultaneously, Niimi navigates persistent advances from track athlete Yukimi Yasuda, forming a love triangle that contrasts Yuri’s reserve.

His family background reveals a father who once played baseball. This connection proves crucial when Niimi’s father intervenes to secure pitcher Eiji Kurahashi’s team participation, persuading Kurahashi’s father of baseball’s role in building resilience. Niimi further mentors Kurahashi, confronting behavioral issues linked to suppressed frustrations over baseball restrictions alongside Karasawa.

Niimi’s deepening baseball dedication parallels his commitment to Yuri. His arc peaks in aiding the team’s push to qualify for the national championship at Kōshien Stadium, marking his athletic and personal growth.