That's it. I've just described for you the basics of what this most excellent anime series is all about. One-Punch Man is the ultimate riff on the whole shonen fighting anime genre. You know the kind of shows I'm talking about: the lone warrior hero starts off against unbelievable odds, faces an unbeatable opponent, then either trains or finds the power within himself to rise to his enemy's abilities, and then returns to take him down after an epic bout of fisticuffs. Lather, rinse, repeat. Think Drag-on BallZ, Bleach, Naruto, One Piece, Fist of the North Star, etc.
One-Punch Man takes that basic concept, but then essentially shits all over it. Things typically start off with monsters, mutants, and super-fighters attacking City Z from out of nowhere, but then our hero Saitama appears (dressed in a super retarded yellow body-suit with red boots and dish-washing gloves, and a huge, flowing white cape) and blows the invading assailants away with only one punch. This is usually done in the most amazing, bloody, and fucking awesomest way possible.
The plot of One-Punch Man goes a little something like this: Monsters and super villains constantly attack the big cities of the world, and one man, sick of the little guy getting stepped on, decides he's going to become the ultimate hero and save humanity. And so he trains super hard for 3 years in order to make his dream come true. He trains so hard in fact that all of his hair falls out. But due to his rigorous body regiment, our titular hero Saitama becomes the ultimate warrior (no offense to James Hellwig). Now, whenever Saitama faces an enemy, it only takes him one punch to defeat the bad guy, much to his own chagrin.
See, Saitama now thinks that he's over-trained himself over the past 3 years, since all he really wants is a fight with an opponent that lasts more than 2 seconds. And so he seeks out the biggest, baddest, toughest evil-doers on the planet (and some from beyond our world) just to tussle with them for hopefully more than one exchange of blows, normally not caring how much collateral damage to the city's infrastructure that he causes.
Soon Saitama impresses a young cyborg warrior with a pure heart, and finds that the boy wants nothing more but to call Saitama "master." Saitama, despite his awesome abilities, is a bit too lazy to care about training anybody though, and he quickly tries to brush this kid, Genos, off. But Genos has seen Saitama's power firsthand, and he deep down needs to learn how to vanquish evil with one swift punch, just like his master. Especially since he's trying to avenge his family and friends who were killed by a nameless cyborg in the past, whom Genos has been hunting ever since his broken body was rebuilt by a "doctor for justice." Anyway, Saitama sees the benefit in having a disciple like Genos when he finds out that the cyborg will throw in for half of his rent.
Oh, and then there's the Hero Association — an organization started to gather heroes of all strengths and powers, uniting them in order to fight evil and chaos. The Hero Association actually pays a salary to the heroes under its umbrella, so Saitama decides that it'd be nice to go pro for a paycheck, and Genos comes with him. The insanely incredible assortment of motley heroes (ranking from lowly street fighters in "C Class," to the ultra world-saving, building-smashing "S Class" big shots) would make the creators of The Venture Brothers applaud and tip their hats to the writers of OPM. From Atomic Samurai and his unbreakable katana, down to Mumen Rider (translated as "License-less Rider"), who is essentially a glorified bike messenger, every hero is unique and fucking amazing.
There are so many jokes and references throughout every episode of One-Punch Man, and I absolutely refuse to ruin any of them. This show openly mocks/satirized every macho fighting anime and manga that has ever been written, and it does it so effortlessly. Saitama's personality itself is unequaled in its portrayal of both excitement over new enemies to fight, and boredom over not having anybody worthy of his attention, not to mention his dedication to his training that made him such a super powerhouse in only 3 years. The man has the constitution and endurance of a herd of oxen!
Initially, after 3 episodes of OPM, I was a little worried that the quick joke of "he defeats his enemies with only one punch" would be used up, and that this series would run out of steam very quickly and die a pretty painful and unfunny death, but that was far from the case. The original writer carefully crafted a world in which super monsters and super heroes fight a never-ending battle for the dominance or safety of humanity every day, and the plot turns and character developments are never allowed to get stale. Once Saitama and Genos fall in with the Hero Association, so many opportunities for new stories and character interactions just explode from the screen!
So the cast is epic, the fights are epic (especially that final battle... He got thrown TO THE FUCKING MOON), and even the background music is typically sweeping and epic... But the last thing I want to touch upon is the greatest thing of all about One-Punch Man: its opening theme song. The opening animation is set to the most amazing Japanese power metal music you've ever heard. It is head-bangingly marvelous, and it matches the feel and tone of the show absolutely perfectly. I actually watched the opening 2 to 3 times before starting each episode. It just so completely gets you in the mood for the super-heroic beatings that you're about to experience.
This was the absolute dumbest piece of chauvinistic bullcrap that I believe that I have ever in my life seen. I wanted it gone from my memory as soon as I started watching it. It is not clever, it is not funny, and it is not entertaining in the least. I wish it and its writers and animators boils all over their privates, and I hope to god that the Rossman never takes any more pictures of me sniffing my hot boss' workout shorts after he goes to the gym, so that he never has any blackmail material to use to make me watch this shitty kind of show ever again.
So you think that Bruce Lee or Jackie Chan are the greatest martial artists of all time? They have nothing on Saitama.
Saitama is the ultimate badass. Saitama kills Titans and gods and alien overlords with one punch. And Saitama looks classy doing it.
If you want to see the best exploding heads ever put to film, watch One-Punch Man. If you want to see the greatest martial artist who ever took down an entire city-sized alien invading ship, watch One-Punch Man. If you want to see anything remotely lame or pussy-like, then do not watch One-Punch Man.