The Strongest Man

A rampaging creature tears through A-City and downs the heroes Lightning Max and Smile Man before a broadcast cuts out. Saitama arrives in time to rescue a girl from the monster, who names itself Vaccine Man and claims it exists to wipe out humanity for poisoning the planet. When it swells into a towering form, a single punch ends it, leaving Saitama shouting in frustration. A flashback then rewinds three years to his unemployed days and his run-in with Crablante, a man mutated by eating too much crab. Sparing Saitama, the creature reveals it is hunting a boy with a huge chin, and Saitama soon protects that child, killing Crablante by tearing out its eyestalk with his necktie.

In the present, a colossal figure named Marugori attacks the city, the result of an experimental steroid built by his mad-scientist brother, Fukegao. The brother panics on spotting Saitama perched atop the giant and orders him crushed, an act that instead kills Fukegao himself. Enraged, Marugori seemingly flattens Saitama, only to hear him remark that absolute power is dull before a punch destroys the giant and levels much of B-City. That night a self-styled car-obsessed monster, Super Custom YO649Z Mk. II, mocks Saitama's bald head and is knocked out at once. The next morning brings a Subterranean invasion that turns out to be a dream, though a real, halfhearted incursion follows and ends the moment Saitama beats their king. Outside Z-City, Genos stands amid rotting cattle and starts walking toward town.

Saitama is introduced as the effortlessly unbeatable hero, dispatching Vaccine Man, Marugori, Super Custom YO649Z Mk. II, and the Subterranean King in turn. The flashback establishes his encounter with Crablante as part of his early history, and the closing shot introduces Genos as he heads toward the city.

King appears in a brief cameo, and the entire Super Custom YO649Z Mk. II sequence is original to the anime, absent from the manga. Several casting nods carry over from other franchises: in the English dub, Christopher Sabat voices Vaccine Man, who resembles Piccolo of Dragon Ball, while Bryce Papenbrook voices Marugori, echoing his role as Eren Yeager in Attack on Titan. The first episode was later released officially on YouTube to mark the series' tenth anniversary.