Nyaago, an average elementary school student in Cat Town, is chosen by the Cat Wizard for his kind, caring, and unselfish nature. He becomes the secret superhero Masked Niyander, transforming upon hearing any plea for help—even from rivals or enemies. His whiskers and tail disappear, his eyes change, and he gains superpowered abilities to fight adversaries and protect the community. Nyaago keeps this identity hidden from most, though his younger sister Miko discovers it, adapts, and is later appointed "Junior Niyander" to assist. His closest companion is Pidori, a pterodactyl he raised, who delivers his transformation costume, aids in rescues, and communicates through vocalizations Nyaago understands.

Nyaago consistently dislikes capsicums and caterpillars, prioritizing compassion and truthfulness. As Niyander, he wields a magic sword central to his crime-fighting, earning widespread admiration but also sparking a rivalry with the blue mountain cat Niyon, who seeks to surpass his popularity without malice. Niyander confronts antagonists like HeHe Manto, a self-proclaimed mountain king wielding fireballs and lightning, and the malevolent Dokuro (Skull King), who seeks world domination and repeatedly tries to unmask him before being permanently defeated and imprisoned. Nyaago also faces the Mouse Wizard, who initially aims to transform Cat Town into a rodent settlement but reconciles after being bested, founding a neighboring town instead. Niyon evolves from rival to trusted ally, achieving recognition. Nyaago balances his ordinary school life with his heroic responsibilities as Cat Town's protector.