There is a girl named Ketsu Yukishima. Ketsu goes to a school that has one class for assassination. Their class is secret, everyone thinks that the bottom of the students go there. If a student has bad grades, the school might send them there thinking that's all they're good for. Some students stay, others make their way out. Ketsu loved the class, and, she has a twin sister. She doesn't go to the same school. In fact, they were separated by birth. So now her sister, Akane, has gone missing.