Training so hard that he went prematurely bald, the part-time hero Saitama has become the strongest man alive. So strong that most enemies are utterly annihilated by a single punch… so strong that he barely feels much of anything anymore. One day he meets a young cyborg who desperately wants to become his disciple after witnessing that power, and his condition for this is for the two of them join the Hero Association together.
An action-heavy and somewhat gory battle shounen parody. Though initially comedy-centric and focused on Saitama, it eventually adopts more of an ensemble structure while introducing hints of drama.
More Information:
aniDB (1, 2)
Crunchyroll
Daisuki
Fansub.co
Wikipedia