One Punch Man is one of the most popular shonen anime in recent memory, along with one of the best-selling manga of all time, because of its unique take on the overpowered superhero format. Protagonist Saitama has trained hard enough to be able to beat any enemy with a single punch, but instead of relishing in his overwhelming power, Saitama instead becomes bored because there's nothing left to challenge him.
Source：https://www.cbr.com/opm-lore-for-new-fans/
