One hundred years ago, monstrous creatures known as Meijuu (溟獣) suddenly emerged from the ocean, staining the waters black and preventing humanity from advancing into the seas. In response, humanity turned to submarines and, with only a faint glimmer of hope, began challenging the Meijuu. Among them is a young man named Ven, who cannot let go of the childhood stories his older brother once told him about the "blue sea." Driven by that longing, he boards the submarine Stier as a Meijuu observer.