The seemingly ordinary and unimpressive Saitama has a rather unique hobby: being a hero. In order to pursue his childhood dream, he trained relentlessly for three years—and lost all of his hair in the process. Now, Saitama is incredibly powerful, so much so that no enemy is able to defeat him in battle. In fact, all it takes to defeat evildoers with just one punch has led to an unexpected problem—he is no longer able to enjoy the thrill of battling and has become quite bored. This all changes with the arrival of Genos, a 19-year-old cyborg, who wishes to be Saitama's disciple after seeing what he is capable of. Genos proposes that the two join the Hero Association in order to become certified heroes that will be recognized for their positive contributions to society, and Saitama, shocked that no one knows who he is, quickly agrees. And thus begins the story of One Punch Man, an action-comedy that follows an eccentric individual who longs to fight strong enemies that can hopefully give him the excitement he once felt and just maybe, he'll become popular in the process. [Written by MAL Rewrite]

One Punch Man 2nd Season's commemorative recap special.

Abandoned as a child by her mother, Aoi Tsubaki has always had the ability to see "ayakashi"—spirits from the Hidden Realm. Shirou Tsubaki, her grandfather who shared the same ability, took her under his wing and taught her how to live with the ayakashi in peace. When her grandfather abruptly passes away, the independent Aoi must continue her college career, armed with only her knowledge in cooking as a means of protection against the human-eating spirits. In hopes that the ayakashi will not turn to her or other unknowing humans as a tasty meal, she takes it upon herself to feed the hungry creatures that cross her path. After giving a mysterious ayakashi her lunch, Aoi is transported to the Hidden Realm, where the ayakashi reveals himself to be an ogre-god known as Oodanna, the "Master Innkeeper." There, she learns that she was used as collateral for her grandfather's debt of one hundred million yen, and that she must pay the price for her grandfather's careless decision by marrying Oodanna. Aoi valiantly refuses and decides to settle things on her own terms: she will pay off the debt herself by opening an eatery at Oodanna's inn. Kakuriyo no Yadomeshi follows the journey of Aoi as she proceeds to change and touch the lives of the ayakashi through the one weapon she has against them—her delicious cooking. [Written by MAL Rewrite]

It is a world replete with dragons and Jushiki sorcerers. Jushiki is a devastatingly powerful formula that can alter the laws of physics, capable of creating enormous TNT explosives or poisonous gas through plasma and nuclear fusion. A down-on-his-luck sorcerer named Gayus joins forces with the beautiful but cruel Jushiki practitioner, Gigina, as bounty hunters pursuing dragons. Then one day, they are presented with an odd request: to serve as security guards for a grand festival put on by Mouldeen, the ruler of their kingdom. It is then that a mysterious serial killing of Jushiki sorcerers begins. (Source: TBS Global Business)