The demon world finds itself in a difficult situation because of the war against heaven.
Rune Saber has managed to seal the demon overlord away.
Karen, the princess of the demon world, consults the overlord's close adviser Lucifer, and together they decide to head for the human realm.
Little does she know that it is all a scheme of Lucifer's devising...

Karen enrolls in the school where Rune Saber, AKA Nozomi Yuzuki goes.
As Karen interacts with Nozomi and her classmates, her feelings begin to change...

[From DLsite English]