Mantaro Muscle is the spoiled son of superhero wrestler King Muscle. After 28 years of peace, the Seigi Choujins' (Muscle League) old enemies regroup and form the Demon Manufacturing Plant (dMp, known in the English version as Destruction, Mayhem and Pain). The Muscle League has lost its edge and are overwhelmed by the young, well-trained fighters. Recognizing their weakness, the Seigi Chojin reopen the Hercules Factory (a school for superheroes) and begin training a new generation of heroes to take on the dMp. At first unwilling, Mantaro (Kid Muscle) is one of the young heroes and defeats his father to prove his readiness to graduate. He and the other new Seigi Choujin defeat several members of the dMp and meet Kevin Mask, who quits dMp when he discovers their lack of honor. They also battle Sunshine and his pupils, who destroy the dMp after developing a renewed respect for the fighting spirit of the Seigi Choujins. The manga continues with the New Generation Replacement Tournament, Mantaro's challenge to master his inherited potential (Kajiba no Kuso Chikara, "burning inner strength" or "the fire"), the return of the Chojin Olympics, a fight with the Demon Seed (a villainous group), a backstory for Robin Mask and a tag-team tournament set in the past. Although the manga begins as a fairly lighthearted, humorous (albeit violent) story, later arcs (the No Respect and Demon Seed storylines in particular) have a darker tone and frequently deal with psychological trauma.
