Years after the Gate to another world first opened in Tokyo, Japan now maintains strict control over the passage, with the Japanese Self-Defense Forces continuing their diplomatic and exploratory missions in the Special Region. The sequel shifts focus from ground operations to the Japan Maritime Self-Defense Force as they navigate the uncharted seas of this parallel world. The story follows Petty Officer Hajime Tokushima, a former chef who now finds himself aboard the submarine Kitashio under the command of Captain Gorou Edajima. Their mission takes an urgent turn when an American journalist is kidnapped in the region's turbulent waters, forcing the naval unit to embark on a high-stakes rescue operation. As they infiltrate the merchant nation of Tinae, they join forces with Princess Primera Luna Avion, a shy but determined royal known for her surprising capacity for drink, the pirate heir Shura no Arch, and the winged Ship Guardian Odette Ze Nebula. Together, this unlikely crew faces raging wars between island factions, pirate fleets, monstrous armored whales and sea creatures, while navigating diplomatic tensions with warring nations that still fight with swords and sails. The narrative follows the first two volumes of the GATE Season 2 light novel series, adapting chapters 1 through 48 of the manga continuation. The sequel introduces a four-year time skip from the events of the original series, with production handled by Studio M2 under director Toru Takahashi, with Tatsuhiko Urahata returning from the first season to handle series composition and scripts. The story explores the clash between modern naval strategy and fantasy warfare, as guided missile submarines navigate political intrigues in a world where radar does not exist, testing the resolve of the JMSDF crew as they fight to complete their mission against both human enemies and mystical forces beneath the waves.