A century after an apocalyptic war devastated Earth, Seo-young Cheon emerges from the serene depths of the sea, donned in her haenyeo diver's suit. Fueled by a conflict with Hae-su, the village's haenyeo leader, she seeks solace once more beneath the ocean's surface. Cast unexpectedly into turbulent waves, Seo-young loses consciousness and descends into a massive underwater tunnel. As she walks through it, Seo-young encounters a mysterious woman at its far end.