"Ultimate Muscle," also known as "Kinnikuman Nisei," is a sequel to the original "Kinnikuman" series, continuing the legacy of wrestling and heroism in a unique, comedic, and action-packed narrative. The story is set in a world where superhuman wrestling, known as "Muscle Wrestling," is a global phenomenon. The protagonist, Mantaro Kinniku, is the son of the legendary wrestler Suguru Kinniku (King Muscle), who saved the Earth from the evil Perfect Origin race. Mantaro, initially reluctant to follow in his father's footsteps, is thrust into the world of wrestling when the D.M.P. (Dangerous Monsters Party), a group of villainous wrestlers, threatens Earth.

Mantaro joins the newly reformed Muscle League, a group of heroic wrestlers dedicated to protecting the planet. Alongside his friends and allies, including Terry the Kid, a skilled cowboy wrestler, and Checkmate, a mysterious and powerful fighter, Mantaro trains and battles to prove himself as a worthy successor to his father. The series features a mix of episodic battles and overarching storylines, with Mantaro and his team facing off against increasingly powerful foes, including the D.M.P. and other rival factions.

The narrative explores themes of legacy, responsibility, and self-discovery as Mantaro grows from a carefree teenager into a capable and determined hero. The series balances intense wrestling matches with humor and character development, often parodying wrestling tropes and pop culture. Key arcs include the Chojin Olympics, where Mantaro competes against wrestlers from across the galaxy, and the battle against the D.M.P., which culminates in a high-stakes showdown to save Earth.

Supporting characters, such as Mantaro's childhood friend Sayaka and his mentor Meat, add depth to the story, providing emotional stakes and comedic relief. The series also revisits characters from the original "Kinnikuman," bridging the gap between the two generations of wrestlers. With its blend of action, comedy, and heartfelt moments, "Ultimate Muscle" delivers a dynamic and entertaining narrative that appeals to fans of wrestling and shonen anime alike.