Since the ancient time, there has been the war between human and demons. Ragno, a low-ranking executive from demon world with great ambition, was given a mission to infiltrate into a school and act as a normal human teacher. His true mission is to find the "Nameless Sword" that is said to be able to grant its owner a power to control a world. On the other hand, there are young girls known as "Ark Caliber" who fight the demons with "Treasured Sword" to prevent them from getting Nameless Sword.
The girls are under an international organization that possesses the Treasure Swords "Kamyu". They fight Ragno while attending his class unknowingly. Meanwhile, another transferred student arrives to Ragno's class and she is another Ark Caliber. Although Ragno's battle is significantly tougher, the presence of the new Ark Caliber increases his chance of obtaining the Nameless Sword. However, even if he intends to fight, as a low-ranking executive, he doesn't have much fighting power.
Being in a difficult situation, Ragno decides to use the monsters that other executives refuse to use in order to lure out the girls. He formulates various kinds of traps for the girls to be corrupted by pleasure. Also, using the chance, he can also increase his power if he uses the demons to impregnate them. Since the girls do not know Ragno's identity, Ragno is able to gain more and more trust from the girls at school.
Soon, Ragno will exercise his plan to realize his ambition...