The one punch man season 1, animated by studio Madhouse was aired from October 5 to December 21, 2015. It is the Anime adaptation of one punch man chapters 1 through to chapter 37.
It follows the initial story telling of Saitama’s adventures. It gives a background of how the universe is structured with monsters and how Saitama suffers from a lack of interest and is bored due to his overwhelming strength.
This is apparent in the first scene only, where Saitama defeats an opponent with just one punch and then screams out in agony, “Another fight finished in just one punch !!???”
The season then follows Saitama’s coincidental encounter with Genos the demon cyborg during his encounter with a mosquito monster who has ravaged several cities and monsters. As Genos struggles with her, Saitama comes into the scene and ends her in one slap, from which point onwards Genos insists that Saitama keep him as his disciple.
The opm season 1 then continues to build upon other characters, the one punch man female characters such as Tatsumaki the terrible tornado, and other S class heroes such as Bang silver fang, etc etc.
The season finale occurs when Boros the destroyer, a being from many galaxies away comes to earth in search of a decent battle with someone who measures up to him in strength. He then meets Saitama and battles him to his heart’s content but Saitama again effortlessly defeats his foe. Boros in his fight with Saitama states that ‘he can sense no bounds to Saitama’s power’
This is the official trailer of the one punch man season 1 , released over 8 years ago :