Twin brothers Nagiharu and Aokaze Kuzumi share a single dream: to play baseball together at Koshien. From their earliest childhood, the bond of that promise shapes everything they do. But as they grow, the gap between them becomes stark. Aokaze, the younger brother, possesses a golden right arm that makes him a once-in-a-generation pitching prospect, scouted by the powerhouse Hosenka Jitsugyo High School, known as Hojitsu. Nagiharu, the elder brother, is a left-handed pitcher who has been unable to escape the shadow of his sibling. In middle school, his inability to match Aokaze's results earned him the reputation of a burden on his own team, and he lost nearly all confidence in his ability.

The turning point arrives when a catcher named Ibuki Hayama transfers into their team. Three years earlier, Ibuki captained the Japanese under-12 national team to a championship and has since been called the vanished genius catcher. He sees something in Nagiharu that no one else has. Under Ibuki's guidance, Nagiharu abandons the orthodox overhand delivery he had copied from his brother and adopts a submarine style, a low, sidearm release that is rare and difficult for batters to read. The change unlocks a new kind of pitcher. Rather than follow Aokaze to the prestigious Hojitsu, Nagiharu chooses to go with Ibuki to Hinazakura High School, a private school that was formerly all-girls and has only just become co-educational. Its baseball program is brand new, and every member of the team is a first-year student with something to prove.

Hinazakura, nicknamed Hina High, competes in the East Tokyo district. The team is a collection of outsiders: Kappei Wakamori, a calm and precise batter; the infield duo Urushibara and Wakamori who develop a knack for turning double plays; the mercurial slugger Satomi; and the struggling Kengo, a former star from a powerful middle school program who has fallen into a slump. Together they form a raw but determined unit. Ibuki, acting as captain and catcher, devises strategies that play to Nagiharu's new submarine delivery, forcing opposing hitters to pound the ball into the ground.

The first major test is a practice game against Shisen Gakusha University High School, a top-four team in the East Tokyo district. The leadoff batter for Shisen Gakusha, a talented first-year named Ataru Yoshizawa, immediately hits a home run off a nervous Nagiharu. But Nagiharu steadies himself. Pitching low and outside, he induces ground ball after ground ball, and the Hina infield turns double plays to keep the score close. In the seventh inning, with two runners on and two outs, Satomi crushes a three-run home run off a split-finger fastball, giving Hina a 3-2 lead. As the game reaches its climax, Nagiharu unveils a new weapon he developed during offseason training: a rising slider, or rising fastball-like breaking ball, that he uses to challenge Ataru in his fourth at-bat.

The team's next opponent is Ryokusen High School, the champion of Chiba Prefecture and the former school of their teammate Kengo. The matchup forces Kengo to confront his own slump and his past. As the tournament progresses, Hina faces Hojitsu, where Nagiharu must finally stand on the mound opposite his brother Aokaze. The brothers' childhood promise to meet at Koshien becomes the emotional core of the series, driving Nagiharu to push beyond his limits against opponents who are bigger, faster, and more experienced.

The cast extends beyond the starting nine. The Golden Generation, the elite players who represented Japan in under-15 competition, includes several key rivals: the fearsome Hojitsu cleanup hitter Gouya, the mercurial Yoshizawa of Shisen Gakusha, and the imposing slugger Moko of the Shuzan lineup, the strongest batting order in East Tokyo. Ibuki's own backstory involves a former battery partner named Mei, a catcher from his middle school days, and an injury that forced him to stop batting right-handed, a secret he is finally forced to confront during a critical at-bat.

The story builds toward the final confrontation against Shuzan High School, the East Tokyo powerhouse. Nagiharu, exhausted and battered, continues to pitch with everything he has, determined to fulfill the promise he made to his brother so many years ago. The series concludes with the outcome of that game, the fulfillment or failure of the twins' shared dream, and the closing of the arc that began when two boys swore to reach Koshien together.