The story begins in 2019, just before the change in historical eras for Japan. A rift to another dimension suddenly opens in the skies above, revealing a "turnabout" alternate world called "Shinkoku Nippon." This parallel version of Japan maintained its militarism and remains stuck in the Showa era. Shinkoku Nippon invades our Japan with its "Genmu" gas weapons (on which our own modern weapons have no effect) and giant humanoid "Garran" weapons. The invaders immediately take over our government and all but conquer Japan. As a result, our Japan never ushers in the Reiwa era.

A decade later, our Japan has been revamped as Genkoku Nippon, a puppet state of Shinkoku Nippon. Under harsh censorship, the once thriving manga, anime, idol, and similar sub-cultures have completely died off ... or so it seemed.

[Source: Anime News Network]