With a stray pitch that completely missed the batter, Eijun Sawamura loses his final middle school baseball game. Frustrated by this defeat, Eijun and his teammates vow to reach the national tournament once they are in high school. But everything changes when a scout unexpectedly invites him to Tokyo's prestigious Seidou High School after seeing the potential in his unusual pitching style. Encouraged by his teammates, Eijun accepts the offer, ready to improve his skills and play at a much more competitive level of baseball. However, now surrounded by a large number of skilled players, Eijun struggles to find his place on the team. He declares that he will one day become the team's ace, but that's only if fellow first year Satoru Furuya doesn't take the title first, with his breakneck fastballs that earn him a coveted spot on the starting roster. With the addition of these talented new players to an already powerful lineup, the Seidou baseball team aims to become the best in Japan, facing off against a number of formidable foes that stand in their way.

The hallowed ground of Koshien Stadium is the field of dreams, where the ambitions of high school baseball players come true. After emerging victorious in the autumn tournament last year, Seidou High School baseball's team has finally earned the right to compete there for the first time in seven years. Beyond the spring tournament looms the battle to decide who is the best team in the nation — the Summer Koshien. With the third-year players due to retire after the summer tournament, the team has to integrate the experience of the seniors and the potential of the newcomers to overcome familiar and new opponents alike and win the coveted national title. Meanwhile, pitcher Eijun Sawamura is as determined as ever to earn jersey No. 1 and seize the position of ace from his persistent rival, Satoru Furuya. As the team prepares for their greatest challenge yet, Sawamura and Furuya carry on their struggle to lead their team to glory and become the star of the game: the true Ace of the Diamond.

One hundred boys, each of them in love with baseball, represent the Seidou High School baseball team. On any team, the Ace is the star of the game. Besides the main heroes, others are striving to attain this position, and each of them has their strengths, weaknesses, and emotional journeys that they have to undertake to become the best that they can be. Ryousuke Kominato is known in Seidou not only for his impressive fielding skills but also as an intimidating presence despite his small stature. While together with his partner, Youichi Kuramochi, they are the cornerstones of the team's defense on the field. Youichi is shadowed by his younger brother, Haruichi, whose admiration for Ryousuke is only matched by his own skills at the sport. Together with Yoichi's roommate, Eijun Sawamura, the players reminisce about their path to Seidou High.

After the National Tournament, the Seidou High baseball team moves forward with uncertainty as the Fall season quickly approaches. In an attempt to build a stronger team centered around their new captain, fresh faces join the starting roster for the very first time. Previous losses weigh heavily on the minds of the veteran players as they continue their rigorous training, preparing for what will inevitably be their toughest season yet. Rivals both new and old stand in their path as Seidou once again climbs their way toward the top, one game at a time. Needed now more than ever before, Furuya and Eijun must be determined to pitch with all their skill and strength in order to lead their team to victory. And this time, one of these young pitchers may finally claim that coveted title: The Ace of Seidou.