Episode 1 - The Strongest Man

 A monster appears in the city and begins to wreak havoc. As it is about to kill a young girl, 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. It transforms into a giant monstrosity, and Saitama destroys it with a single punch.

 The episode then switches to a flashback, showing an unemployed Saitama's encounter three years previously with Crablante, 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 Crablante with permanent marker while he was asleep.

When Crablante 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, a colossal man attacks the city. The man is the result of an experimental steroid called "Biceps King", developed by his mad scientist older brother.

 The two set out to conquer the world, only for the older brother to observe, in shock, that Saitama is riding on his brother's other shoulder. He orders the giant to kill the man on his shoulder, accidentally causing his own death. The enraged younger brother attacks Saitama, seemingly defeating him, and bemoans the fact that despite now being the strongest man alive, he feels empty.

Saitama, emerging unharmed, comments that absolute power is boring and defeats the giant, accidentally destroying City B. The next morning, he wakes up to find himself in the middle of a 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 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.