Living in a superhuman society, it is hard to feel special. Even more so when the spotlight only shines on professional heroes, those legally authorized to use their special powers known as Quirks in public for the greater good.

Kouichi Haimawari grew up aspiring to be a hero, but with a mediocre Quirk like "sliding" that ties him to the ground, he soon came to the conclusion that he could only ever admire them from below. Despite this, Kouichi finds contentment in using his Quirk to carry out day-to-day good deeds, such as returning lost items and helping the elderly cross the street.

However, Kouichi's tame life takes a swing into the turbulent when he is rescued from a back alley brawl by Vigilante, or illegal hero, Knuckleduster. Seeing hero potential in Kouichi, Knuckleduster enlists his help in tracking down the source of a dangerous drug known as Trigger that boosts the user's Quirk at the expense of their rationality.

Set in a time before the events of the original story, Vigilante: Boku no Hero Academia Illegals follows Kouichi as he chooses to don the reputation of a villain and become a Vigilante, operating in the shadow of the law to prevent crimes from taking to the surface.