"My Hero Academia: Vigilantes" is a spin-off manga set in the same universe as "My Hero Academia," focusing on the lives of individuals operating outside the sanctioned hero system. The story takes place several years before the events of the main series and explores the underground world of vigilantes—people who use their Quirks to fight crime without official hero licenses. The narrative centers on Koichi Haimawari, a young man with a modest Quirk called "Slide and Glide," which allows him to hover slightly above the ground. Despite his unremarkable ability, Koichi becomes an unlikely vigilante, driven by a sense of justice and a desire to help others.

Koichi's journey begins when he encounters Kazuho Haneyama, also known as Pop Step, a street performer with a Quirk that enhances her jumping ability. Together, they form an informal vigilante duo, patrolling the streets of Naruhata Ward to protect civilians from petty crimes and low-level villains. Their activities attract the attention of Knuckleduster, a mysterious and gruff former pro hero who becomes their mentor. Knuckleduster, whose real name is Iwao Oguro, trains Koichi and Pop Step while pursuing his own vendetta against a dangerous criminal organization experimenting with Quirk-enhancing drugs.

The plot delves into the darker aspects of hero society, exploring themes of corruption, inequality, and the moral ambiguity of vigilantism. As the story progresses, Koichi and his allies face increasingly dangerous threats, including Trigger, a drug that temporarily boosts Quirk abilities, and the shadowy figures behind its distribution. The narrative also introduces recurring characters from the main series, such as Eraser Head (Shota Aizawa) and Midnight (Nemuri Kayama), providing insight into their lives before they became prominent pro heroes.

Character development is a key element, with Koichi evolving from an aimless college student into a determined and resourceful vigilante. Pop Step struggles with her aspirations to become a hero and her growing feelings for Koichi, while Knuckleduster's tragic past and relentless pursuit of justice add depth to his character. The series also explores the lives of ordinary citizens and the challenges they face in a world dominated by Quirks, offering a more grounded perspective compared to the main series.

The manga balances action, drama, and humor, with a focus on the personal struggles and relationships of its characters. It expands the "My Hero Academia" universe by highlighting the lives of those who exist on the fringes of hero society, providing a fresh and compelling narrative that complements the main storyline.