At a private girls' boarding school, Myōjō Academy, thirteen girls are transferred into the academy's "10th Year Class Black" (10年黒組, Jūnen Kurogumi). Of these thirteen students, twelve are assassins from various backgrounds who are all tasked with assassinating the remaining student, a girl named Haru Ichinose. If they succeed, they will be granted any wish they desire, but if they fail, they are expelled from the class. However, one of the assassins, Tokaku Azuma, has sworn to protect Haru from the other assassins.