On August 20XX, a gate suddenly appears in Tokyo's Ginza district, unleashing a portal where monsters, medieval knights and other fantasy beings come from another world and wreak havoc on Tokyo. The Japanese Defense Force take action against these monsters and push them back into the "Gate". Third Reconnaissance Team is dispatched to the "Special Region" lead by officer (and otaku) Youji Itami. On their travels, they are joined by a beautiful elf girl who is a survivor from the dragon's rampage and guide the group across the dangerous new world.

[Source: Crunchyroll]

The second season of the Gate anime adapts the Enryuu-hen and Douran-hen arcs.

Some time has passed since humanity drove back the invading forces from what is now known as the "Special Region." In that time, a bustling town has developed within the protective walls of the SDF basecamp, and relations continue to improve favorably.

However, some in the Kingdom still do not trust Japan, viewing them as a dangerous entity, bent on invading and taking over. Although Itami and the others are working hard to make peace with the Kingdom, dark forces are secretly at work attempting to force Japan's hand. Gate: Jieitai Kanochi nite, Kaku Tatakaeri Second Season focuses on the ongoing political struggle between Japan and the "Special Region" just beyond the Gate.

[Source: MyAnimeList]

Thanks to the efforts of Lieutenant Yoji Itami (now Lieutenant First Class) of the Japan Ground Self-Defense Force, the once-closed "GATE" has safely reopened. The Japanese government, determined to maintain exclusive control over the "GATE", continues mobilizing the Self-Defense Forces for political and topographical surveys to build a peaceful relationship with the inhabitants of the Special Region. Captain First Class Edajima Goro of the Japan Maritime Self-Defense Force (JMSDF) and his subordinate, Petty Officer Second Class Tokushima Hajime, a skilled former chef, were conducting such diplomatic and investigative missions, traveling across the various nations of the Special Region.

However, one day, their assignment suddenly escalates when they're tasked with a special mission to rescue an American journalist kidnapped on an island in the Special Region's Blue Sea.

Immediately boarding the JMSDF Oyashio-class submarine Kitashio, they set sail into the ocean of another world, discovering the area around the Blue Sea engulfed in turmoil, with archipelago nations and pirates battling fiercely for dominance.

[Source: official English website]