Based on the game by Triangle Bitter.

The depths of our planet hide a subterranean, magical world named Roa currently in the grip of a terrible war between the all-female knight corps of Queen Gloria and the monstrous troops of the evil syndicate Zerou.
The game starts when Metzer, a young and brilliant Zerou officer, gets official approvation from Zerou's high command to try a new plan suggested by the protagonist himself. Secretly relocated to the surface of Earth, his mission consists in gaining the trust of the Sweet Knights, the two japanese girls invested by Queen Gloria with the power to protect the surface world from the monsters of the syndicate. Next, once discovered their weak points, Metzer will use his monsters to defeat, kidnap and brainwash the Sweet Knights into sex slaves devoted to Zerou's cause.
However the syndicate do not know (yet) of Metzer's great ambitions...