"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 individuals with unique abilities, known as "Hero Skills," are enrolled in specialized academies to hone their talents and protect the world from threats. Hiroshi, despite lacking a powerful or flashy ability, possesses an unyielding determination and a strong sense of justice, which drive him to pursue his goal.

The narrative begins with Hiroshi enrolling in the prestigious Hero Academy, where he faces numerous challenges due to his seemingly ordinary skill set. Unlike his peers, who wield extraordinary powers like super strength, elemental manipulation, or advanced technology, Hiroshi's ability is modest—enhanced reflexes and heightened endurance. This disparity often places him at a disadvantage, but his resourcefulness and strategic thinking allow him to compete with more gifted students.

Throughout the series, Hiroshi forms bonds with classmates who each have their own motivations and struggles. Among them is Aiko, a prodigy with the ability to manipulate energy, who initially underestimates Hiroshi but gradually comes to respect his perseverance. Another key character is Riku, a rival with a tragic past, whose immense power is both a blessing and a curse. The dynamics between these characters are explored as they navigate the rigorous training, intense competitions, and moral dilemmas inherent in their path to becoming heroes.

The plot is structured around the academy's curriculum, which includes simulated battles, rescue missions, and theoretical studies. These events serve as both training exercises and tests of character, pushing the students to confront their limitations and grow stronger. As the story progresses, external threats emerge, including rogue individuals with dangerous abilities and organizations seeking to exploit heroes for their own gain. Hiroshi and his friends are forced to confront these challenges, often blurring the line between right and wrong.

A recurring theme is the definition of heroism itself. While many characters initially equate heroism with power and fame, Hiroshi's journey emphasizes qualities like empathy, teamwork, and self-sacrifice. His actions inspire those around him, challenging the notion that strength alone determines a hero's worth. The series also delves into the societal pressures and ethical responsibilities that come with being a hero, exploring how individuals balance personal ambitions with the greater good.

The narrative builds toward a climactic arc where Hiroshi and his peers face a formidable antagonist whose motives are deeply tied to the flaws within the hero system. This confrontation forces the characters to reevaluate their beliefs and confront the complexities of their chosen path. The resolution underscores the importance of resilience and integrity, leaving a lasting impact on both the characters and the world they strive to protect.