Tokaku Azuma has just been accepted into Myoujou Academy, a prestigious private ladies' boarding school. But there's a catch: she's an assassin, and she's taking part in a challenge to assassinate their sweet-natured classmate, Haru Ichinose, with 11 of her classmates in Class Black. Regardless of the hardship or expense, whoever achieves will be awarded their truest wish. Each assassin, however, has only one chance to kill her; if they fail, they will be exiled.