The heroines has some eroge powers, like hypnotism, being invisible to other that function like time stop, common sense alteration, etc. Those powers are actually foreign, and try to corrupt the heroines, they are kinda like a catalyst, so the powers can invade the world. And the protagonist has to stop them from using their powers, but he can't let them know that he knows they have those supernatural abilities, or else they will corrupt faster. 

PS. The heroines personalities at the beginning is really unbearable, their personality is outright tr*sh, but the author managed to write a romantic story using such personalities