In the world of , nearly everyone in the world has powers known as "Quirks". Normally, citizens cannot use their Quirks out in the open—doing so is a privilege reserved by law only for registered superheroes. However, a stubborn few have chosen to reject this rule. They are known as vigilantes: people with the hearts of heroes, but the reputation of villains.

19-year-old Kouichi Haimawari always dreamed of being a hero but didn't think he had a good enough Quirk. Instead, he would spend his free time sliding around town doing simple good deeds, like picking up litter or helping lost children find their parents. Content with his lot in life, Kouichi never expected to be thrown into the world of heroes, but his life is changed forever by an unexpected run-in with Knuckle Duster, a vigilante, an illegal hero.