A monster appears in the city and begins to wreak havoc. As it is about to kill a young girl, One Punch Man arrives and saves her. The monster introduces itself as Vaccine Man, and claims to have come into existence to eradicate mankind due to their pollution of Earth. Vaccine Man then transforms himself into a giant monstrosity, and One Punch Man proceeds to kill him with a single punch, after which he screams in frustration at his own strength. The episode then switches to a flashback, showing an unemployed Saitama’s encounter three years previously with Crabrante, a man who gained crab-like features after eating too much crab. Crabrante spares him, explaining that it is hunting a boy with a large chin. Saitama promptly encounters the child, who is revealed to have drawn nipples on Crabrante with permanent marker while he was asleep. When Crabrante finds them, One Punch Man decides to protect the child, and kills the monster by ripping out its eyestalk with his necktie. As present-day One Punch Man reminisces about the events, a colossal man attacks the city. The giant man named Marugori (lit. “Beefcake”), is the result of an experimental steroid called “Biceps Brachii King”, developed by his mad scientist older brother named Fukegao (lit. “Oldface”), who first appears riding on Marugori’s right shoulder, saying that this was incredible. The episode reverts to a flashback of Marugori having a workout in his older brother’s lab, while Fukegao (who have just done inventing his steroid drug “Biceps Brachii King”) ingested the drug to his little brother, claiming that this makes him the most powerful man on earth. Marugori immediately drinks it and grows rapidly at a high rate. Fukegao, who was just standing watching him growing fast remarks that this will do. The episode jumps back to the present-day Marugori, who was now a severely mutated giant, with Fukegao’s arrogant statement that The Brain and the Brawn Brothers would have the greatest of mind and the strongest of the bodies and also remarked that they would rule the world as kings. The giant then swing his arm and the city was destroyed in an instant. Fukegao, observing, was astounded by the death of so many people that he motivates his younger brother to destroy the next town. Moments later, One Punch Man reappears riding on Marugori’s other shoulder, the reason that Fukegao was shock. He orders Marugori to kill the guy on his shoulder, accidentally causing Fukegao’s own death when his brother smacks the wrong shoulder. When Marugori investigates his own hand, he then freaks out in shock, saying that why this just happens to him and he just wants to be strong and when he saw One Punch Man on his left shoulder, he begins to enrage. The enraged Marugori first throw One Punch Man to the ground and he then jumps back with all of his raging heart and attacked One Punch Man with various blows, which leaves a giant hole. Marugori mourns for Fukegao after seemingly crushing One Punch Man and bemoans the fact that despite being the strongest man alive, he feels empty. One Punch Man, emerging unharmed, agrees that absolute power is boring and defeats the giant with a single punch, accidentally destroying City B when the giant falls on it. After One Punch Man’s victory, he runs into a strange being looting an abandoned area. The monster reveals itself to be the Super Custom YO649Z Mk.ll, a being that loves custom cars to the point of wearing car parts in order to resemble one. One Punch Man takes out the monster with a single punch when it mentions his bald head. The next morning, he wakes up to find himself in the middle of an invasion of the surface by overpopulated Subterraneans, who have already exterminated 70% of humanity. Surprised by their strength, he deems them worthy opponents and engages them in a mighty battle. At the moment he has defeated a great number of them and is about to fight the Subterranean King, he wakes up to discover that it was a dream. Finding an actual invasion by the Subterraneans outside, he is disappointed when they quickly surrender after he easily defeats their king. After the credits, Genos is seen standing in a field of desiccated cow corpses. He then turns and begins walking towards the city.