After saving a child, Saitama trains hard for years to become a hero.

The mysterious organization known as The House of Evolution creates a monster that moves toward City Z; Genos, a young cyborg, attempts to stop it, but gets in over his head before Saitama happens along.

Saitama and Genos find out about the House of Evolution from Armored Gorilla and learn that Genus wants Saitama for his experiments.

A terrorist group arrives in F City and is led by a large and battle-suit wearing person named Hammerhead.

Saitama decides to become a professional hero and takes the test with Genos; Saitama barely passes while Genos achieves high scores.

Since becoming a professional hero, Saitama lazes around the house until Genos tells him that inactive heroes get dropped from the registry.