"Mezase Hero!" centers on the journey of a young boy named Hiroshi, who dreams of becoming a professional hero in a world where heroes are officially recognized and trained. The story is set in a society where heroism is a regulated profession, with heroes undergoing rigorous training and certification to combat threats and maintain peace. Hiroshi, an ordinary boy with no extraordinary powers, enrolls in the prestigious Hero Academy, where he must compete against peers who possess unique abilities and talents.

The narrative explores Hiroshi's struggles and growth as he navigates the challenges of the academy. Despite his lack of innate powers, Hiroshi relies on his determination, strategic thinking, and resourcefulness to keep up with his classmates. His journey is marked by intense training sessions, competitive exams, and missions that test his physical and mental limits. Along the way, Hiroshi forms alliances and rivalries with other students, each with their own motivations and backstories. Key characters include Aya, a skilled combatant with a mysterious past; Takeru, a prodigy with a cocky demeanor; and Rina, a supportive friend who shares Hiroshi's ideals of heroism.

The plot delves into themes of perseverance, teamwork, and the true meaning of heroism. As Hiroshi progresses through the academy, he uncovers deeper layers of the hero system, including its flaws and the moral dilemmas faced by those in power. The story balances action-packed sequences with moments of introspection, highlighting the personal growth of its characters. By the end of the series, Hiroshi's journey culminates in a climactic showdown that challenges his resolve and redefines his understanding of what it means to be a hero.