One Punch Man follows the adventures of Saitama, a nihilistic unemployed man who decides to become strong in order to become a superhero. Trouble is he becomes too powerful and he has the ability to kill any enemy with a single punch. On paper this sounds like it would be awesome, right? The ability to decimate any foe with just one blow? If I might digress for a moment, let's talk video games for a second. This probably dates me a bit, but back in the day there used to be something known as "God Mode" which was essentially a cheat code you could input into some games to make your character invincible. At first, it would be fun, traveling around destroying everything without worrying about dying. But very quickly, the game would become monotonous and boring. The challenge was gone. This is exactly what Saitama is feeling--he has nothing to test his abilities.
What makes One Punch Man so fantastic is that it works on many levels. On the surface the show operates as a legit Seinen anime albeit a parody of one. There are plenty of the tropes inherent in the genre: super strong beefcake guys, obsession with power levels, teacher/student relationships and over-the-top battles. Even if you only accept it on this basic surface interpretation it's still outstanding. The animation is excellent, the character designs are fun and the writing is hilarious. Although it's satire, it doesn't really deconstruct the genre as much as it pokes light-hearted fun at its themes. If you choose to dig deeper then there is definitely more going on at its heart though.
Even though Saitama is most likely the most powerful being in the universe he is not happy. His ennui permeates every aspect of his life and he is unable to feel personal satisfaction at his accomplishments. He takes on an apprentice, a headstrong cyborg named Genos and even that doesn't quell his lust for recognition. Saitama represents every person who has ever been stuck in a dead-end job that they hate--even if they excel at that job. I imagine that many Japanese people feel a bit trapped as many roles are forced on them after they graduate school. Get a job. Find a place to live. Start a family. All these life goals can be stressful. Saitama joins a sort of union for superheroes and ends up having to do busy work to rank up. Even within supposed "heroes' there are people vying for attention and special treatment.
While this all sounds rather depressing (and it is for Saitama, unfortunately) the people around him step up their game because of his power, and are better for it. Genos idolizes him and learns to believe in himself and his own strength. Other heroes who are lower in the superhero hierarchy fight with gusto even though the odds are stacked against them. It's all quite noble and touching when everything is said and done. Though Saitama might never get his wish to fight a truly powerful opponent, people are still saved from certain death by his actions. Genos and Saitama represent the dual nature of the workplace. Genos loves his job and wants to be come the best. Saitama is already the best, but his accomplishments don't fill the void he feels inside.
There is a glimmer of hope, however. By the end of the series it seems as if Saitama does learn to enjoy his feats, even if the feeling is faint and fleeting. It's all about perspective. If you only get one punch, you better make it count for something.