Kid Muscle (or Mantaro for short) is a bumbling, gross, and wimpy sort-of person. Unfortunately, he has been summoned to protect the earth from the evil dMp, a band of cheating wrestlers from outer space. Luckily he's not undertaking this task alone. Kid Muscle must join the Muscle League, a group of wrestlers from outer space who are training themselves so they can defend the earth, and stop the dMp by acheiving Ultimate Muscle, a hidden power that makes Mantaro an almost unbeatable opponent.

Source: ANN

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Prequel: Kinnikuman: Kinnikusei Oui Soudatsu Hen
Other: Kinnikuman II-sei
Side story: Kinnikuman II-sei: Muscle Ninjin Soudatsu! Choujin Daisensou
Sequel: Kinnikuman II-sei: Ultimate Muscle