Chapter 18: Business Activity

Just five days into their careers as registered heroes, Genos asks whether he can move in with Saitama. Saitama objects until the cyborg pledges to cover rent and drops a thick wad of cash on the table to seal it. Once settled, Genos begins jotting down Saitama's every action, convinced that careful observation will unlock the source of his mentor's strength. Saitama, meanwhile, quietly schemes about how to get his new roommate to leave.

His plotting is cut short when Genos mentions a rule Saitama never knew: a C-Class hero who stays inactive for over a week is dropped from the Hero Association. Alarmed, Saitama bolts out to track down some crime to handle. He tears across the city but comes up empty and eventually quits for the day. He repeats the desperate search the following day, panicking about expulsion, until he instinctively snatches a thrown kunai out of the air. The weapon belongs to Sonic, whose name Saitama has already forgotten.

Genos becomes Saitama's roommate by offering to pay rent and begins documenting his behavior to study his power. Saitama discovers that C-Class heroes are removed after a week of inactivity and frantically searches the city for crimes to stop. On his second day of searching, he catches a kunai flung by Speed-o'-Sound Sonic, whose name he cannot recall.

This is the 18th chapter of the One-Punch Man manga, collected in Volume 3 within the National Superhero Registry Arc, and it corresponds to webcomic Chapter 18. Its events are adapted in Episode 6 of the anime. The bonus page shows a flock of crows feeding on Beast King's corpse, after which they mutate into small human-shaped creatures able to handle eating utensils.