"My Hero Academia: Vigilantes" is a spin-off series set in the same universe as "My Hero Academia," taking place several years before the events of the main series. The story centers on Koichi Haimawari, a young man with a minor Quirk called "Slide and Glide," which allows him to hover slightly above the ground. Unlike the heroes in training at U.A. High, Koichi operates as a vigilante, using his Quirk to help people in Naruhata, a district of the city, without official hero licenses.

Koichi's life changes when he encounters Kazuho Haneyama, also known as Pop☆Step, a high school student and aspiring idol with a Quirk that enhances her jumping ability. Together with their friend, the mysterious and eccentric Knuckleduster, a former pro hero turned vigilante, they form an unlikely team to combat crime and protect their community. Knuckleduster, who lacks a Quirk but compensates with immense physical strength and combat skills, serves as a mentor figure to Koichi, pushing him to refine his abilities and confront his limitations.

The narrative explores the gray areas of hero society, focusing on the struggles of those who operate outside the official system. The trio faces various threats, including street-level criminals, rogue Quirk users, and organized crime syndicates. One of the primary antagonists is the villainous group known as the "Instant Villains," individuals who gain temporary, unstable Quirks through a drug called Trigger. This drug becomes a recurring element in the story, highlighting the societal issues surrounding Quirk usage and regulation.

As the story progresses, Koichi's Quirk evolves, revealing hidden potential that sets him apart from ordinary vigilantes. His growth as a character is paralleled by his deepening relationships with Pop☆Step and Knuckleduster, as well as his interactions with other figures in the hero world, including Eraser Head (Aizawa) and Midnight, who appear in supporting roles. The series also delves into the backstories of its main characters, particularly Knuckleduster, whose tragic past and personal vendetta add emotional depth to the narrative.

Themes of justice, morality, and the consequences of operating outside the law are central to the story. The series balances action-packed sequences with moments of introspection, exploring the motivations and struggles of its characters. While "My Hero Academia: Vigilantes" maintains ties to the main series through shared characters and references, it stands on its own as a compelling exploration of the hero society's underbelly, offering a fresh perspective on the world of heroes and villains.