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 Chojin Crown, the tournament which Mantaro (Kid Muscle) reluctantly participates in. After going along with the obstacles (most of which were created to try and eliminate him), he qualified.

Mantaro has made it to the semifinal match of the Chojin Cup. He must now fight Ricardo in a soccer field to advance to the last match. Kevin Mask is also in the semifinals, and must defeat Ilioukhine in an aerial field to advance. These three matches will determine who wins the Chojin Cup.

Set in a far flung medieval-looking world of Arst, Prince Jordy Volder takes up the fight against the ambitions of the conqueror Marder. Jordy uses the legendary giant robot "panzer" Galient, which is one of many panzers that have been preserved underground for thousands of years. Using an army of advanced robot panzers, Marder is conquering all of Arst in preparation of his plan for dominance of the Crescent Galaxy.