Nami Shimaoka, a central figure, works as a designer on humanity's ocean city project. Discovered as an infant in a storm-driven shipwreck and placed ashore by humanoid sea creatures, she was raised in Ibusuki by an elderly couple.

Unbeknownst to her surface associates, she secretly serves as an agent for an ancient undersea civilization. Her mission involves sabotaging the ocean city construction to halt its severe ecological damage caused by massive concrete pollution. This role positions her as the prime suspect after a sudden explosion occurs at the construction site where she and protagonist Hiroshi Umino are present.

Following the explosion, she rescues Hiroshi and his companion Yamamori from drowning, then disappears. She leaves Hiroshi a note indicating her return to her "snowy hometown," prompting him to travel to Ibusuki seeking her. Her true allegiance lies with the sea people's queen, Izanami, and she actively participates in their resistance against surface humanity, viewed as an invasive force destroying the marine environment.

As the conflict escalates, she rises to military leadership within the undersea civilization, commanding forces in attacks against human naval assets, including ships and submarines. During the climax, she returns to the undersea palace to report battle setbacks and witnesses the civilization's evacuation plan. Ultimately, she survives the underwater city's destruction, ascending into space with the surviving remnants of her people.