Kinnikuman II Sei takes place several years after the events of the original Kinnikuman. Mantarou Kinniku is the 59th prince of Planet Kinniku and son of the renowned wrestler, King Suguru. Lazy, immature, and cowardly, Mantarou seems to have little in common with his heroic father. When a powerful group calling themselves the dMP threatens the Earth, only someone as powerful as Kinnikuman stands a chance against them. Not knowing the current whereabouts of the King, a plan is formed to have his son Mantarou take up the mantle. The idea of fighting super villains does not sit well with Mantarou, who initially runs away from his duty. He flees to Earth where he encounters Meat, his father’s old trainer. Despite Mantarou's shortcomings, Meat sees King Suguru in him, and believes he can set the prince on the right path. Thus begins Mantarou's journey to better himself and save the world.

The episode is set during a school festival where Shinobu is assigned to write a script and make outfits for her class play. Youko and Alice notice that Shinobu is sleepy every morning and are worried that she might be getting tired of all the work. Will Shinobu be able to successfully complete all her tasks in time? (Source: MAL News)

Long ago, the king of Orvelia, Kyle, defeated the demon king Angmund and brought peace to the world. However, one hundred years later, demons have been sighted in the forests, threatening humanity once more. A scouting expedition is sent to verify the claim but only one member returns. Meanwhile, Riheet, the leader of a dark elf mercenary group, plots to take over Orvelia to exact revenge against the humans who forsook their race a century ago. The knight apprentice Kasel, accompanied by the priestess Frey, sets out to rescue Clause, an old friend who went missing in the tragic mission. However, along the way, Kasel discovers that he is the son of the revered King Kyle and the only one who can wield the Holy Sword Aea—the same sword that slew the demon king. To fulfill this destiny, the young knight must embark on a perilous quest, unseal the sword, and end the fear instilled by demons. As Kasel's journey to bring hope to humanity and Riheet's vow for vengeance intertwine, what fate could possibly await them?

Mari and Kinnikuman's Championship Belt are stolen by the Octopus Dragon and his minions. Now Kinnikuman and the other Justice Choujins must travel to Planet Metro and compete in a tournament in order to win them back.