Three years before the setting the start of the series, Saitama, with a full head of hair, has an encounter with the Mysterious Being Crablante. Saitama stands before Crablante, who seems surprised that he is not running in fear like the other people, Saitama sighs and Crablante comes to the conclusion that Saitama is a salary man and due to the fact that he isn't running, he must want to die. Saitama agrees with that but states that he is, in fact, unemployed and is on a job hunt and that he had already been rejected from a job that morning. Saying that he has lifeless eyes like himself, Crablante decides to let him off the hook, stating that his real prey is a child with a cleft chin.
Crablante walks away and Saitama notices a child playing with a soccer ball in the park next to him, a child with a cleft chin. Saitama asks the kid if he did anything to a crab monster and the kid agrees, saying that he drew nipples on the monster with a permanent marker. At that moment Crablante appears behind Saitama and goes to attack the child, but Saitama dives and saves the kid. Saitama says the kid needs to run, but the kid says he wants his ball. Crablante is confused, as to why Saitama is trying to save the child, and that at this point it's too late to stop because he'd butchered some people on his way there, and that he'll do the same to anyone who makes fun of him. Crablante is in the middle of a rant when Saitama starts laughing, stating that Crablante reminded him of a villain from an anime he'd watched as a kid at which point Crablante hits him and sends him flying. Crablante goes to attack the child and is about to do so, but gets hit in the head with a stone thrown by Saitama. Saitama states that he remembers his dream as a child had been to be a hero who could beat any villain with one punch, and he throws off his coat and wraps his tie around his hand and challenges Crablante. Crablante starts to beat Saitama up, hitting him once, twice, but before he can hit him a third time, Saitama loops his tie around Crablante's eye stalk and rips it out of his head, killing him.
Three years later Saitama, now super-powered and bald, is recalling the events of those days and that even though he obtained an unbeatable power he is still unsatisfied.