SUMMARY
A monster appears in A-City and starts wreaking havoc as it defeats Lightning Max and Smile Man. The reporter tells what happened and then the report is suddenly cut and a bald man named Saitama decides to leave. As the monster is about to kill the girl, incoming Saitama comes and saves her. The monster introduces himself as Vaccine Man and claims that he was created to wipe out humanity due to their pollution of the Earth. Vaccine Man then transforms into a giant monster and Saitama destroys him with a single punch. After killing the monster with a single blow, Saitama let out a frustrated yell.
The episode then flashes back to three years before he became a hero, showing the unemployed Saitama meeting Crablant, a man who acquired crab-like features after eating too many crabs. The monster spares him, explaining that he is hunting a boy with a big beard. Saitama immediately encounters a child who is revealed to have drawn permanent marker on Crablant’s nipples while he was sleeping. When Crablante finds them, Saitama decides to protect the child and kills the monster by ripping out its eye socket with his tie. While 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 watch in shock as Saitama rides on his brother’s other shoulder. He orders the giant to kill the man on his shoulder, accidentally causing his own death. Enraged, the younger brother attacks Saitama, seemingly defeating him, and laments that he feels empty despite now being the strongest man in the world. Emerging unharmed, Saitama comments that absolute power is boring and defeats the giant with a single punch, accidentally destroying B-City.
After defeating the giant, he is returning home at night when he is spotted by the Mysterious Being and revealed to be a Super Custom YO649Z Mk. II. While the Super Custom YO649Z Mk. II brags about his love for building custom cars so much that he dresses up and turns into a custom car loving monster, Saitama just stares at him and the monster insults his bald head, prompting Saitama to hit him with a single blow and kill the monster.
He wakes up the next morning to find himself in the middle of an invasion of the surface by overpopulated Underground People who have already wiped out 70% of humanity. Surprised by their strength, he considers them worthy opponents and embarks on a mighty battle. Just as he has defeated a large number of them and is about to fight the Underground King, he wakes up to find that it was only a dream. Finding an actual invasion of Subterraneans outside, he is disappointed when they quickly surrender after easily defeating their king and they display a white flag that reads, “I’m sorry for that”. Outside Z-City, Genos, who was first shown standing in a field of decomposing cow corpses, turns and starts walking towards the city
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