Mabuse, a Japanese giant salamander dwelling in the underwater Sea Kingdom, harbors jealousy and bullies Sirius, the kingdom's heir. He covets the Eye of Argon, the powerful artifact Sirius protects that signifies rulership. Mabuse attempts to seize it, claiming the burden is too heavy for Sirius, but Sirius refuses and easily defeats him.

After failing to convince Sirius, Mabuse deduces Sirius hides something and secretly follows him. He witnesses the wise sea turtle Moelle reveal to Sirius a potential path to reach a heavenly star using spores from Klaesco blossoms on Mobius Hill during an eclipse, allowing Sirius and Malta to be together. Mabuse immediately confronts them, denounces them as traitors, and swims off to report this to King Glaucus.

Later, Mabuse approaches Sirius's younger brother, Teak, proposing a dangerous plan to free the imprisoned Sirius: Teak must steal the Eye and bring it to the prison, supposedly enabling Sirius's escape during the chaos. Mabuse conceals his true aim to seize the Eye. When Teak reluctantly presents the plan, Sirius rejects it. After Teak retrieves the Eye, Mabuse ambushes him with his gang, steals the artifact, and drunkenly celebrates, declaring himself the new king. Teak eventually recovers the Eye during the celebration. Mabuse's actions directly contribute to the chain of events causing Argon's temporary release and the resulting destruction.