"Kinnikuman Nisei - Ultimate Muscle" is a sequel to the original "Kinnikuman" series, set several decades later in a world where professional wrestling is dominated by superhuman beings known as Chojin. The story centers around Mantaro Kinniku, the son of the legendary wrestler Suguru Kinniku (Kinnikuman), who is reluctantly thrust into the world of Chojin wrestling to protect Earth from various threats. Mantaro initially lacks the drive and discipline of his father, but as the series progresses, he grows into his role as a hero and wrestler.

The narrative begins with Mantaro living a carefree life, unaware of his family's legacy. However, when the evil Chojin group known as the DMP (Demolition Man Party) threatens Earth, Mantaro is forced to join the ranks of the Muscle League, a federation of heroic Chojin wrestlers. Alongside his friends and allies, including Terry the Kid, Checkmate, and Gazelleman, Mantaro trains and competes in high-stakes matches to defend humanity. The series features a mix of comedic moments, intense wrestling battles, and dramatic character development as Mantaro learns to harness his latent abilities and embrace his destiny.

The plot is structured around various wrestling tournaments and arcs, each introducing new adversaries and challenges. Notable arcs include the "Chojin Olympics," where Mantaro and his team face off against powerful opponents from across the galaxy, and the "Survival Match," a brutal elimination tournament that tests the limits of the Muscle League. Throughout these events, Mantaro's relationships with his teammates and rivals evolve, highlighting themes of camaraderie, perseverance, and self-discovery.

Key characters include Mantaro Kinniku, whose growth from a reluctant hero to a confident leader forms the emotional core of the series; Terry the Kid, a skilled and loyal friend who often serves as Mantaro's rival and motivator; and Kevin Mask, a recurring antagonist with a complex relationship to the Kinniku family. The series also features appearances from characters of the original "Kinnikuman," bridging the gap between the two generations and enriching the lore of the franchise.

"Kinnikuman Nisei - Ultimate Muscle" blends action, humor, and heartfelt moments, offering a continuation of the "Kinnikuman" legacy while introducing a new generation of characters and conflicts. The series maintains a balance between episodic wrestling matches and overarching storylines, making it accessible to both new viewers and fans of the original series.