The main character, Tokak Azuma, trained on a hitman and is one of the best students.Her teacher Kaiba chooses Woman as a representative of their school, which must be transferred to Mёdzё school and trained in Kurogumi group. Nevertheless, he thinks Kaiba Tokak worst student. Kurogumi group in which to learn the girl - a special class, formed for a short time.The last time he did not last two weeks. In this class student 12 murderers, and their sole purpose, Haru Itinose, the destruction of which is to form Kurogumi. In the classroom, there are three rules. The first is the one who can kill Haru, receive any award that wish. Second means are irrelevant,but to involve anyone other than members of the group can not Kurugumi. Third, the failed assassin or violates the second rule will be excluded. Acquainted with Haru, Tokak begins to feel strange to her to feel. Moreover, Haru is able to correctly answer the questions Kaiba, for which she could not answer Tokak.As a result, it saves Tokak Haru from the first attempt, and after learning about her sad past, decides to become her protector.