Kizuku Ōyama commands the experimental submarine SS-99. He leads the vessel and its crew on missions to investigate the enigmatic Ocean Empire and confront its military leader, Hell Deathbird. His critical task involves the search for the missing marine biologist Dr. Juzou Oki and Dr. Oki's grandson Gorou, who vanished during an underwater investigation in the Japanese Trench. Under Ōyama's authority, the SS-99 battles Ocean Empire forces, engaging hostile submarines like the X-187 and X-0.