The Deep Sea King
A pack of monsters calling themselves the "Clan of the Seafolk" arrive on the shores of City J intent on invading the land. When the hero sent to fight them is defeated easily by the Deep Sea King himself, Saitama and Genos head for the city to help.
Aired 23/11/2015EP 8 - The Deep Sea King
2015
One-Punch Man
Saitama has a rather peculiar hobby, being a superhero, but despite his heroic deeds and superhuman abilities, a shadow looms over his life. He's become much too powerful, to the point that every opponent ends up defeated with a single punch.
The lack of challenge has driven him into a state of apathy, as he watches his life pass by having lost all enthusiasm, at least until he's unwillingly thrust in the role of being a mentor to the young and revenge-driven Genos.