Several years have passed since the Gate, a portal to another world, suddenly opened in Ginza, Tokyo. From it came armed knights, foot soldiers, and strange beasts that wreaked havoc on our world.

Although the Gate was once sealed, it was later reopened through the efforts of First Lieutenant Yoji Itami of the Japan Ground Self-Defense Force (JGSDF). It now remains under the exclusive and strict management of the Japanese government.

Since then, the JSDF has been dispatched to the Special Region, the vast world on the other side of the Gate, to take part in cultural exchanges with the locals and conduct surveys. Because this land holds its own unique cultures and environments, the JSDF must act carefully to avoid disrupting its balance.

Meanwhile, Petty Officer 2nd Class Hajime Tokushima, a former chef now serving in the Japan Maritime Self-Defense Force (JMSDF), travels under the command of Captain Goro Edajima across the Avion Sea. Aboard the Kitashio, an Oyashio-class submarine, they navigate distant waters untouched by Japanese influence.

Their mission is to infiltrate the trading nation of Tinae and rescue an American journalist who has been abducted in the area.

Along the way, they encounter Princess Primera Luna Avion of Tinae, Shura no Arch, a descendant of a family of righteous pirates, and Odette Ze Nebula, an Avi maiden who serves as Ship Guardian. Before long, the five begin working together as a team.

However, the region is plagued by war, as neighboring island nations and pirate fleets clash for dominance. To make matters worse, the seas are home to Armored Whales and other unknown creatures, pulling the crew into struggles beyond their imagination.

Faced with challenge after challenge, will Tokushima, Edajima, Princess Primera, and their companions overcome the dangers before them and complete their mission?