"Meimon! Daisan Yakyū-bu" centers on the Third High School Baseball Club, a group of students striving to make their mark in the competitive world of high school baseball. The series highlights the challenges and camaraderie within the team as they navigate their way through rigorous training, personal growth, and the pressures of competition. The narrative focuses on the players' determination to overcome their underdog status and prove themselves against more established teams.

The protagonist, Takaya Todoroki, is a passionate and skilled pitcher who joins the Third High School Baseball Club with dreams of leading his team to victory. His dedication and leadership qualities inspire his teammates, but the road to success is fraught with obstacles. The team lacks the resources and reputation of their rivals, and internal conflicts often arise as the players grapple with their individual insecurities and ambitions.

Key characters include Todoroki’s teammates, each with distinct personalities and roles on the team. Among them is Shun Mitaka, a talented but aloof batter who struggles with teamwork, and Yūki Hoshino, a hardworking catcher who serves as the emotional backbone of the group. The dynamics between the players are central to the story, as they learn to trust one another and work cohesively despite their differences.

The series delves into the rigorous training regimens and strategic planning required to compete at a high level. Matches are depicted with a focus on the tension and excitement of the game, showcasing the players' growth both on and off the field. The narrative also explores the personal lives of the characters, highlighting how their experiences outside of baseball influence their performance and relationships within the team.

As the Third High School Baseball Club progresses through tournaments, they face increasingly formidable opponents, each match serving as a test of their resolve and teamwork. The series emphasizes the importance of perseverance, unity, and self-belief, as the players strive to achieve their shared goal of becoming a respected and successful team. Through their journey, "Meimon! Daisan Yakyū-bu" captures the essence of high school sports, blending intense competition with heartfelt moments of friendship and personal development.