A Dangerous Being

The chapter opens on Fukegao, who has just perfected his greatest creation, a steroid he calls Biceps Brachii King. He feeds the drug to his younger brother Marugori, promising it will make him the mightiest man alive. Marugori's body swells the instant he drinks it, growing until he bursts through the ceiling and towers over the nearby hills. He plucks his brother from the debris, sets him on his shoulder, and Fukegao declares the pair the Brain and Brawn Brothers, the strongest and cleverest duo in the world. The giant marches on the closest settlement, D-City, and erases it with one sweep of his arm before his brother orders him toward the next target.

News networks broadcast the unfolding catastrophe as panic spreads across the country. On the way to B-City, Fukegao notices a stranger perched on Marugori's opposite shoulder, and that stranger is Saitama. Marugori misjudges which side the intruder is on and swats the wrong shoulder, killing his own brother by mistake. Realizing he has crushed Fukegao rather than Saitama, the giant unravels, wailing that all he ever wanted was more power and blaming the death on Saitama. He snatches him up, slams him into the ground hard enough to leave an enormous crater, and pounds him deeper, believing he has won. As Marugori confesses the hollow emptiness he feels even at his strongest, Saitama agrees, springs out of the pit, and ends him with one punch to the face. The corpse crashes down onto B-City, wrecking it in the process.

Fukegao's Biceps Brachii King steroid transforms his brother Marugori into a colossal giant, and the pair name themselves the Brain and Brawn Brothers. Marugori destroys D-City in a single swing. Saitama rides unseen on the giant's shoulder, and Marugori accidentally kills Fukegao while trying to strike him. Enraged, Marugori batters Saitama into a crater, only for Saitama to rise and kill him with one punch, his falling body destroying B-City.

This is the third chapter of the series and falls within the Saitama Introduction Arc, printed in Volume 1 and released on June 21, 2012. Its events are adapted in Episode 1 of the anime. Both Fukegao and Marugori die over the course of the chapter.