Captain Gorou Etajima and cook Hajime Tokushima set out aboard a submarine to rescue an American journalist abducted across the Avion Sea, a stretch of the Special Region split between rival island states and pirate fleets. Adapting the light novel sequel to Gate, this manga follows the Japan Self-Defense Force's naval branch into a part of the alternate world the original story never covered. Diplomatic tension and old rivalries complicate the rescue as the crew crosses contested waters.