Mantaro comes from Muscle Planet. His family history is dedicated to wrestling. Mantaro's father, Suguru Muscle (a.k.a. Kinnikuman/King Muscle) , had entered almost every tournament to become a champion. But after 28 years of peace, the Seigi Choujins' (Muscle League) old time enemies regrouped and formed the dMp (Demon Manufacturing Plant, although this is never mentioned in the English dub. There, it is simply referred to as Destruction, Mayhem & Pain) and crashed landed at Mount Fuji. (The dMp are a parody of the wrestling faction, the nWo. The dMp's T-shirts are virtually identical in basic design to those of what are now considered classic nWo shirts: the letters dMp in white and surrounded by a single line forming a white box.) Time and the years without evil to fight have taken their toll on the old Muscle League, who are overwhelmed by these youthful and well-trained fighters.
Realizing that they have lost their edge, the Seigi Chojin reopen the Hercules Factory, the training school for superheroes, and begin training a new generation of heroes to take on the dMp. Although unwilling, Mantaro (Kid Muscle) is one of the young heroes who takes part and ultimately defeats his father to prove his worthiness to graduate. He and the other new Seigi Choujin fight and defeat several members of the dMp and meet Kevin Mask (Parody of popular wrestler Kevin Nash), who quits the organization after seeing that they lack honor. They also battle Sunshine and his pupils, who destroy the dMp after developing a rekindled respect for the fighting spirit of the Seigi Choujins.