70% of the Earth is ocean, and that 70% has become humanity’s enemy. The seas are now ruled by powerful creatures known as Meijuu, or Demon Beasts, whose sudden appearance has made oceanic travel impossible and severed diplomatic relations between countries. Despite this, humanity refuses to give up entirely, launching submarine-based attacks against the Meijuu in a desperate struggle to reclaim the waters.

The story follows Ven, a young man who cannot abandon the dream of a blue sea his brother described to him in childhood. While most view the ocean with fear, Ven longs to see it as the beautiful expanse his brother promised. To pursue this vision, he joins the submarine Steer as a Meijuu observer, a position that places him at the frontlines of humanity’s conflict with the sea’s new masters. The central question driving the narrative is what color the sea will ultimately reflect in Ven’s eyes. Set in a world where the ocean has transformed from a source of connection and life into a deadly barrier, the manga follows Ven’s journey aboard the Steer as he confronts the terrifying reality of the Meijuu while holding onto the idyllic memory of the sea his brother spoke of.