First volume of One Punch Man (ワンパンマン), a Japanese manga created by a person with the pseudonym One.
On an unnamed Earth-like super-continent planet, strange monsters and villains have been mysteriously appearing and causing disasters. To combat them, the world's superheroes have risen to fight them. Each of the Hero Association's superheroes are ranked from Class-C to Class-S. Saitama is one such hero, hailing from the metropolis of City-Z. He has trained himself and grown strong to the point that he can effortlessly defeat any opponent with a single punch. However, Saitama became a hero for the fun of the experience, and he has lately become bored with his superhuman power and frustrated at the complete lack of strong opponents that can challenge him.