Episode 1

"The Strongest Man" is the first episode of the One-Punch Man anime series.

Summary
A monster appears in A-City and begins to wreak havoc as he defeats Lightning Max and Smile Man. A reporter tells what has happened and then the report was suddenly cut out and a 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 screamed in frustration.

The episode then switches to a flashback three years before he became a hero, showing an unemployed Saitama's encounter 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 a 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 while shopping, a colossal man attacks the city. The man is the result of an experimental steroid called "Biceps Brachii 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 B-City.

After defeating the giant, he returns home at night when he was noticed by a mysterious being, revealing himself to be Super Custom YO649Z Mk. II. While Super Custom YO649Z Mk. II brags about his 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 the monster insults his bald head, prompting Saitama to strike him with a single punch, killing the monster.

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 where 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 defeats their king and they show a white flag that says, "Sorry About That". Outside Z-City, Genos who was first shown standing in a field of decaying cow corpses, turns and begins walking towards the city.

Trivia

 * King makes a cameo in this episode.
 * Crablante gives the episode monolog at the end of the credits.