One Punch Man starts off by having a large potentially copyright infringing purple monster man going on a destructive rampage, claiming to be the wrath of the Earth itself to cleanse the humans from the planet so that it is no longer tainted with their polluting ways. Before this beast can slay a little girl, our hero Saitama leaps in to her rescue. He confronts this beast who proceeds to taunt Saitama, who retaliates by punching the monster, causing it to explode with the singular hit. Saitama, however, is not pleased, for there was no challenge in it. With his power being so immense, there was just no thrill and excitement in his heroism. His life continues on in this way, every opponent he faces is defeated with a singular punch without the foes even being able to do a single thing to hurt Saitama, causing him depression and boredom.

Eventually, Saitama meets a cyborg by the name of "Genos" who is a hot-blooded young man seeking a violent cyborg who happened to destroy his hometown along with his family. After an exciting encounter with a mosquito lady, Genos finds himself in awe of Saitama's strength and forcibly becomes Saitama's student so as to find the secret to his sheer power.

However, a mysterious group of individuals also take an interest after their mosquito lady was destroyed by him, for they wished to harness it for themselves. And just how did Saitama happen to gain his insane strength? He did basic strength training for three years. Every day, 100 push-ups, 100 sit-ups, 100 squats, and a 10km run as well as eating three square meals, getting adequate sleep, and not using the AC or heat in winter and summer. That's it.

Going ahead a bit, Saitama finds out that despite his heroic acts and utter might, nobody knows who he is, breaking his spirit a bit in a comical scene. Genos then informs Saitama of the Hero Association, an officially ran society of superheroes who are paid for heroics and dispatched to the various monsters and dangers that present themselves to the world in order to restore peace. Together, they join this association where they take both a physical and a written exam. Genos scores 100 points, earning him an S rank membership out of the gate, whereas Saitama got only a barely passing 71 points, earning him C rank, breaking his heart a bit. From here on, we see the various interactions between people of the Hero Association and the villains that attack the world, as well as the odd danger such as an asteroid, showing how the world is, all the while Saitama just has poor luck, never getting the credit due to him, and even gaining a negative reputation once the aforementioned asteroid is destroyed by him, since the pieces of it scattered around destroying much of the city it was going to annihilate. Though Saitama uses this reputation to his advantage in a later scene when Saitama shows up last to fight a fearsome opponent, and one of the on-lookers brings up why it seems odd how all these heroes that threw themselves at this mighty monster all were swatted aside and nearly died for nothing, doing no damage to it whereas Saitama just blew him apart with a single punch. The rest of the on-lookers start agree with him until Saitama proverbially throws himself under the bus, boasting and saying that he'll get all the credit since everyone else "weakened" the monster first but he finished it off, and since he already had a bad reputation, they believed that he was trying to just steal credit and cheat his way up the ranks and that maybe the other heroes really did do something after all.

Getting away from the show's plot, this show has a lot of great over-the-top action and hilarious moments, with some of the best stuff coming from the expressions on people's faces. The music is also pretty great. You can listen to the theme below. The environments look pretty dang snazzy, and the effects used in the show all compliment the situations they're used in. There's never really a dull moment, just carrying on from one anecdote to another, or going onto a fantastic over-the-top action scene usually chased by another joke. And as a nice touch, you can notice a lot of returning characters in the background such as a little tyke who had his balloon rescued from a tree by the hero Mumen Rider returning in a later scene to cheer Mumen Rider on.