

Saitama, the protagonist, is pretty much the most powerful hero around, but since he's not doing it for fame he didn't even know about the association and nobody really knows him. That's where the game gets interesting though.

Heroes can rank up by completing missions and tasks, but ultimately it also depends on popularity. Divided by range, these heroes are cataloged depending on their abilities and heroic activities. In this universe, it's not rare to see monsters attacking the cities and people, so they created a Hero Association to fight the evil menace. He's pretty much submerged in a dull lifestyle, and even though he's the strongest hero, nobody really knows him. But as time passes he gets really bored, as now there are no challenges in life, no threats, he starts losing all his emotions. Now he's such power that he can defeat any enemy with just one punch. He trained every day until he lost all his hair and became bold. One-Punch Man tells the story of a guy who wanted to be a hero for fun.
