Goro Oki is the elder grandson of Dr. Juzou Oki, the prominent marine biologist who designed the advanced submarine Super 99. He works as deputy officer at the Oki Ocean Lab, assisting his grandfather in marine research operations. Goro's involvement starts when baffling attacks strike the Japanese Trench. He accompanies Dr. Oki on a survey mission aboard the exploration submarine Kuroshio to investigate these disturbances. Before departure, Goro entrusts his younger brother Susumu with a modified Type 99 rifle, a significant family heirloom.

During the deep-sea expedition, the Kuroshio is attacked and captured by the Ocean Empire forces under General Hell Deathbird. Goro and Dr. Oki are taken hostage aboard the enemy vessel. Their disappearance sparks the central conflict, forcing Susumu to take command of the Super 99 submarine. Goro endures prolonged imprisonment by the Ocean Empire, held in close quarters with its leadership, Queen Ze Violencia and General Hell Deathbird. His status as a captive becomes a driving force for the Super 99 crew's campaign against the Ocean Empire and their mission to rescue the researchers.

Goro's storyline resolves when the Super 99 crew defeats the Ocean Empire forces and secures his release alongside Dr. Oki. Following their rescue, both Oki men return to their marine research work.