Thanks to the efforts of Lieutenant First Class Yoji Itami of the Japan Ground Self-Defense Force, the previously sealed "GATE" has been safely reopened. The Japanese government, committed to keeping exclusive authority over the "GATE," continues deploying the Self-Defense Forces for political and geographical surveys aimed at establishing peaceful relations with the inhabitants of the Special Region. Captain First Class Edajima Goro of the Japan Maritime Self-Defense Force and his subordinate, Petty Officer Second Class Tokushima Hajime—a talented former chef—were carrying out such diplomatic and reconnaissance assignments, journeying across the various nations of the Special Region.
But one day, their mission unexpectedly intensifies when they are given a special assignment: rescue an American journalist who has been kidnapped on an island in the Special Region's Blue Sea.
Boarding the JMSDF Oyashio-class submarine Kitashio without delay, they set sail into the ocean of another world. There they discover that the area around the Blue Sea is engulfed in chaos, with archipelago nations and pirates locked in fierce conflict for control.
