Hundreds of years have passed since humankind was destroyed by "Aka" beasts that appeared out of nowhere
Dr. Georg and his assistant Alicia are living alone and conducting research of producing "new human beings" that would be able to eradicate the monsters.
But shortly before the prototype "Mizuki" is completed, the lab is attacked by a powerful armed group. Mizuki is going on a journey in search of a powerful monster to extract the necessary organ to save his creators and future humankind...

[From vndbreview]