Eijun Sawamura, now a second-year student, steps onto the mound carrying the coveted ace number one jersey for Seido High School's baseball team. This summer tournament in West Tokyo represents the final chance for third-year captain and catcher Kazuya Miyuki, along with their senior teammates, to reach the national stage at Koshien. The season opens with the fifth round of the All-Japan High School Baseball Championship, where Seido faces Norikane Academy, and Sawamura delivers a dominant performance, showcasing the growth and newfound reliability that earned him the ace role.

The narrative follows Seido's journey through the West Tokyo tournament bracket, interwoven with critical matchups between rival schools. The ferocious slugger Raichi Todoroki and the Yakushi High School team battle the genius pitcher Kosei Amahisa and Ichidai Sankou in a tense quarterfinal that ends in heartbreaking defeat for Yakushi. Meanwhile, the defending powerhouse Inashiro Industrial, led by the charismatic ace Mei Narumiya, crushes its opposition to claim a semifinal spot. Seido advances past Sosei High School in the quarterfinals, setting up a highly anticipated semifinal showdown against Ichidai Sankou.

The semifinal between Seido and Ichidai Sankou becomes the emotional and dramatic centerpiece of the season. Sawamura and Amahisa engage in a gripping pitchers' duel, trading zeroes through the early innings. Seido's defense bends but does not break, even as first baseman Kenta Maezono commits a costly error. The game hinges on small tactical battles: safety bunts, stolen bases, and perfectly executed relays. Veteran reliever Norifumi Kawakami struggles with elbow pain, forcing the coaching staff to make difficult decisions about the pitching rotation. Sawamura's resolve deepens with every inning, drawing on the trust built with Miyuki and the lessons learned from his early failures. The season builds toward the final innings of this semifinal, with the outcome determining who will face Inashiro for the West Tokyo championship and a ticket to Koshien.

Key characters include Sawamura, whose eccentric personality and relentless work ethic drive his evolution from a wild rookie to a composed ace; Miyuki, the sharp-minded catcher and captain whose last summer adds urgency to every play; Satoru Furuya, Sawamura's quiet and powerful rival pitcher, who pushes him to improve; Yoichi Kuramochi, the speedy and aggressive shortstop; Haruichi Kominato, the reliable second baseman with a keen batting eye; and new first-year players such as catcher Koshu Okumura and slugger Masashi Yuki, who inject fresh energy into the team. The setting shifts between the Seido dormitory, the practice fields, and the stadiums of West Tokyo, particularly Jingu Stadium, where the intensity of single-elimination tournament baseball plays out. The season explores themes of inherited legacy, the weight of the ace number, the pain of injuries, and the unspoken bonds between teammates who know this may be their last game together.