Saitama is a guy who’s a hero for fun. After saving a child from certain death, he decided to become a hero and trained hard for three years. Though he’s now so strong he can defeat any opponent with a single punch, lately he feels as if, in exchange for overwhelming power, he’s lost something even more important.

Saitama and Genos glean information about The House of Evolution and its founder Dr. Genus, a scientist with dangerous ideas on artificial human evolution. Meanwhile, Dr. Genus learns of Saitama’s amazing strength and has plans of his own for One-Punch Man.

A story set sometime between Mosquito Girl’s defeat and Genos showing up to Saitama’s apartment. Genos wants to learn the secret to Saitama’s power and decides to follow him and to observe everything he does. At the same time, Saitama is convinced that someone is stalking him.