"Kinnikuman Nisei - Ultimate Muscle 2" continues the story of the next generation of superhuman wrestlers, set in a world where wrestling is a global phenomenon and superhuman abilities are the norm. The series centers around Mantaro Kinniku, the son of the legendary wrestler Suguru Kinniku (Kinnikuman), as he strives to live up to his father's legacy while carving out his own path in the wrestling world. Mantaro is a carefree and somewhat lazy teenager, but his latent potential as a wrestler becomes evident as he faces increasingly formidable opponents.

The narrative begins with Mantaro and his friends forming the new generation of the Muscle League, a wrestling organization dedicated to protecting Earth from evil forces. The league is composed of a diverse cast of characters, each with unique abilities and personalities, such as Terry the Kid, a skilled marksman; Checkmate, a chess-themed wrestler; and Gazelleman, a swift and agile fighter. Together, they face off against the DMP, a malevolent organization seeking to dominate the wrestling world and conquer Earth. The DMP is led by the sinister Kevin Mask, a former ally turned antagonist, who harbors a deep grudge against the Kinniku family.

As the story progresses, Mantaro and his allies participate in high-stakes wrestling matches, often with the fate of the world hanging in the balance. These matches are characterized by over-the-top action, strategic combat, and the use of signature moves that reflect each wrestler's unique abilities. The series explores themes of friendship, perseverance, and the struggle to define oneself in the shadow of a legendary parent. Mantaro's growth as a wrestler and as a person is a central focus, as he learns to balance his natural talent with hard work and determination.

The plot also delves into the history of the Kinniku family, revealing secrets about Suguru's past and the origins of the Muscle League. This backstory adds depth to the narrative, connecting the events of the series to the broader "Kinnikuman" universe. The rivalry between Mantaro and Kevin Mask serves as a driving force, culminating in intense battles that test both characters' resolve and abilities.

Throughout the series, the stakes escalate as the DMP introduces increasingly powerful and dangerous wrestlers, forcing Mantaro and his friends to push their limits. The narrative balances humor, action, and emotional moments, creating a dynamic and engaging story. By the end of the series, Mantaro emerges as a true hero, having proven himself worthy of the Kinniku name while forging his own identity as a wrestler. The conclusion ties up major plotlines while leaving room for future adventures in the "Kinnikuman" universe.