Episode 1

The Strongest Man (最強の男, Saikyō no otoko) is the first episode of the One-Punch Man anime series.

Summary
A monster appears in A-City and begins to wreak havoc. Lightning Max and Smile Man try to defeat it but were utterly defeated. A man in the news report tells what has happened and then the report was suddenly cracked out and the bald man named Saitama decides to leave. As the monster is about to kill a girl, an incoming Saitama 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 Saitama destroys him with a single punch. After a single punch killed the monster, Saitama frustrates and screamed in frustration.

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. The monster 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 a permanent marker while he was asleep. When Crabrante finds them, Saitama decides to protect the child, and kills the monster by ripping out its eyestalk with his necktie. As present-day Saitama reminisces about the events while shopping, a loud and a noisy sound distructs the people inside the grocery store, all running away except Saitama who was just standing still and behind him was the entrance of the grocery store being stepped off, destroyed it and Saitama then investigates what happened and the camera reveals a giant footprints of a gigantic creature who attacks the city. The gigantic creature was revealed to be the colossal man. The man (who's name was Marugori) is the result of an experimental steroid called "Biceps Brachii King," developed by his mad scientist older brother (named Fukegao) ,who first appears riding on the man's right shoulder. The episode reverts to a flashback of Marugori having a work out in his brother's lab. Fukegao, who had just done making his ultimate steroid drug "Biceps Brachii King" handed this to Marugori, telling him that this will obtain the power that he has always been seeking for. He then drinks it and yells out in pain and his body grows in size at a rapid rate. Fukegao, who was just only watching him growing, remarks that this will do. The episode jumps back to the present-day Marugori, who was now a severely mutated giant, walking on the city with his older brother narrating him that The Brain amd the Brawn Brothers (by combining the greatest of the minds and the strongest of the bodies), will conquer everything on earth and said that they would rule as kings. When Marugori destroys the city with a single swing, Fukegao was astounded by the death of so many people that He then motivates Marugori to destroy the next town. As Marugori continues to wreck the city, almost all people evacuating following the emergency announcement, Fukegao continues to command his brother to scatter them like rats. Moments later, Fukegao notices that Saitama is riding on Marugori's other shoulder. He orders the giant to kill the man on his shoulder, accidentally causing Fukegao's own death. When Marugori discovered that Fukegao is dead, he then freaks out, calling for his beloved brother and remarking how did this just happened to him then said that he just wanted to be strong and finally the strongest man. When he saw Saitama on his left shoulder, He then enrages by grabbing Saitama, telling him that he doesn't know who he is and also stated that his brother is dead and throwing him to the ground instantly, destroying a building and jumps back and creates a massive assault, thinking that Saitama would be defeated by the hands of his strong punches. What it leaves from his massive punch assault was a gigantic hole. When Marugori seemingly defeats him and bemoans the fact that despite now being the strongest man alive, he feels empty. Saitama, emerging unharmed, punches Marugori's face, killing him and comments that having an overwhelming strength is pretty boring then the giant on the other hand, lands to the B-City, accidentally destroying it.

After defeating the giant, he returns home at night when he was noticed by a mysterious being, who was previously looting him and then appeared before him, revealing himself to be Super Custom YO649Z Mk. II. While Super Custom YO649Z Mk. II tells his own information that he love to build custom cars so much that he decked himself out and turned into a custom car-loving monster, Saitama just stares at him and kills him with a single punch, causing the demise of that monster. After this, Saitama was shown washing his track suit and taking a bath in his bathtub.

The next morning, he wakes up to find himself in the middle of a invasion of the surface by overpopulated Subterranean People, who have already exterminated 70% of humanity. Surprised by their strength, he deems them worthy opponents and engages 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 only a dream. Finding an actual invasion by the Subterraneans outside, he is disappointed when they quickly surrender after he easily defeat their king and showing a white flag that says, "Sorry About That". Outside Z-City, Genos who was first shown standing in a field of decaying cow corpses and he then turns and begins walking towards the city.

Characters in order of appearance

 * 1) King (Cameo)
 * 2) Vaccine Man
 * 3) Lightning Max
 * 4) Smile Man
 * 5) Saitama
 * 6) Agoni's Grandchild
 * 7) Crabrante
 * 8) Marugori
 * 9) Fukegao
 * 10) Super Custom YO649Z Mk. II
 * 11) Subterranean King
 * 12) Genos

Episode Notes

 * King makes a cameo in this episode.
 * Crabrante gives the episode monologue at the end of the credits.