Dr. Juzo Oki leads the Oki Ocean Lab as its director and chief architect of the experimental submarine SS-99, named in homage to his family’s Type 99 rifle. The submarine pioneers an innovative "L-type engine" that eliminates radiation emissions, while its revolutionary design enables seamless transitions from underwater navigation to aerial flight in later models. Subsequent upgrades expand its capabilities to interstellar travel three decades after its maiden voyage.

While investigating mysterious shipwrecks near the Japan Trench, Juzo and his son Goro face an ambush by the hostile Ocean Empire’s submarine. Their disappearance during the mission catalyzes a chain of events, leading his grandson Susumu to inherit a cryptic key to the SS-99’s concealed facility.

Captured by the Ocean Empire, Juzo is compelled to engineer military technology for their arsenal, including the formidable Z-type aircraft-carrying submarine. This forced collaboration pits his ingenuity against the ambitions of Hell Deathbird, the empire’s ruthless military commander, showcasing the exploitation of his technical prowess.

Juzo’s legacy persists through the SS-99’s core systems, which evolve into a spacefaring vessel by the series’ conclusion, underscoring the enduring influence of his innovations long after his era of active contribution.