is set on an isolated, rural, and peaceful island on the Seto Inland Sea called Torishirojima, which has a population of about 2,000 people. The protagonist, Hairi Takahara, grew up in an urban setting, but after an unpleasant incident, he uses the recent death of his grandmother as an excuse to come to the island to take care of her estate sale. Once there, he gets to know four girls, who are the focus of the story: Shiroha Naruse, who forgot her summer vacation; Ao Sorakado, who is pursuing the legends of the island; Kamome Kushima, a high-class girl looking for a pirate ship; and Tsumugi Wenders, a younger girl trying to find herself.

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.

What will happen when a desperate boy who grew up among evil people steps into the world of cultivating immortals? Immortals and demons can be separated by a thought, and life and death are not decided by heaven. Let's see how he goes against the current in this world where humans and demons coexist and evil spirits run rampant, and become a legend.

Six days and nights, a strange case, and the boy returning to twelve years ago, can he break the shackles of the Kingdom of God this time?