The shipgirls carry within them the souls of naval vessels from history—and they are the only ones who can fight the Deep-Sea Dwellers, a fleet of hostile vessels born from the ocean floor. Overwhelmed and losing ground, the fleet assigns destroyer Shigure to a new unit: the First Ranger Unit's Third Division, a mismatched group built around the aging battleships Yamashiro and Fusou, the aircraft cruiser Mogami, and four destroyers. Their mission is a decoy operation designed to cover the main force—a last stand by ships with little left to lose.