This story is about "Puellarium", a school isolated from the outside world by fog and thorns. Here strict discipline is always in effect, the air always tense, and time flows undisturbed. However, one morning a girl was found unconscious in front of the school gate. This gate is always closed and should only open on special occasions. Viewed suspiciously by the students and asked her to name herself, she replied: "Alouette."

At that moment, the bell of the clock tower, which was rusty and always silent, rang out. "When the bell of the clock tower rings, the end of this school will come soon." Like the gossip had hinted, it was the beginning of a nightmarish turmoil in Puellarium. Alouette who had became a student after many complications, violated curfew and left her room. What she saw there was a school that had turned into a different world, and strange misshaped beings that wandered around with a murderous intent. And finally, a girl who stood resolutely to face them in combat.