Inumimi Berserk

Rihito has a younger sister, Yuro. One day, dog-ear-like objects suddenly appear on her head and she refuses to go to school. He somehow takes her to school by using the carrot and the stick. However, when they get home, they find their house being destroyed and in rubble. When they are at a loss at what to do, a woman suddenly attacks them. Just when they are about to get killed, Hazumi, Yuro's classmate, saves them. Why do they get involved in a battle...?