Being just a normal student without a special talent, Makoto Naegi wins a lottery to attend the prestigious Hope's Peak Academy where only the top prodigies attend. However, instead of this being the beginning of a wonderful high school life, it's a ticket to despair, because the only way to graduate from Hope's Peak Academy is to kill one of your fellow students or be one of their victims.

In 2071, roughly fifty years after an accident with a hyperspace gateway made the Earth almost uninhabitable, humanity has colonized most of the rocky planets and moons of the Solar System. Amid a rising crime rate, the Inter Solar System Police (ISSP) set up a legalized contract system, in which registered bounty hunters, also referred to as "Cowboys", chase criminals and bring them in alive in return for a reward.

Saitama is a hero who only became a hero for fun. After three years of “special” training, though, he's become so strong that he's practically invincible. In fact, he's too strong — even his mightiest opponents are taken out with a single punch, and it turns out that being devastatingly powerful is actually kind of a bore. With his passion for being a hero lost along with his hair, yet still faced with new enemies every day, how much longer can he keep it going?

After the fall of the Galactic Empire, lawlessness has spread throughout the galaxy. A lone gunfighter makes his way through the outer reaches, earning his keep as a bounty hunter.