Plot

Saitama is a guy who's a hero for fun. After saving a child from certain death, he decided to become a hero, training hard for three years.

Saitama is a guy who is a hero for fun. After saving a child from certain death, he decided to become a hero and trained hard for three years. Though he's now so strong he can defeat any opponent with a single punch, lately he feels as if, in exchange for overwhelming power, he's lost something even more important.—Minhaj

A monster who calls himself "Vaccine Man" displays his destructive capabilities by destroying parts of a major city. As he is about to kill an innocent young girl, the self proclaimed "Hero for fun", Saitama manages to rescues her. Vaccine Man explains his hatred for the humans who have polluted the environment and plans to completely wipe out humanity. Vaccine Man transforms from a humanoid like figure into a distorted monster, attacking Saitama only to be destroyed effortlessly with a single punch. Enraged by his immense strength, insight is given into the life of Saitama three years earlier, where he aspired to become a hero after saving a young boy from the crab monster, Crablante. Further looking back on those three years where he was unsatisfied with being a businessman, Saitama claims that he trained so hard to become a hero that he went bald. Shortly after, the Brain and Brawn Brothers: Fukegao the mad scientist and Marugori, now a transformed giant due to his brother's enlargement drink invade and destroy much of the city. After accidentally killing his own brother, an angered Marugori rampages against Saitama, who is able to withstand his attacks without injury. Defeating Marugori with a single punch, Saitama walks back to his apartment for sleep and unknowingly dreams about facing the subterranean race, who pose an actual challenge to him. After destroying many of the underground invaders, Saitama is awoken by his alarm clock, ultimately waking up to a disappointing reality. He looks outside his window to bear witness to the real life subterranean race, only they are much weaker than in his dream and immediately retreat after Saitama defeats their leader with ease.