Tokaku Azuma has just transferred to Myoujou Academy, an elite private girls' boarding school. However, there is a catch: she, along with 11 other students in Class Black, is an assassin participating in a challenge to kill their sweet-natured classmate, Haru Ichinose. Whoever succeeds will be granted their deepest desire, regardless of the difficulty or cost. However, each assassin gets only one chance; if they fail to kill her, they will be expelled. Despite the extraordinary reward, Tokaku decides to take a different course of action. Although Haru is her target, the young assassin soon finds herself drawn to the very girl she is supposed to kill. With the entire class out for Haru, Tokaku refuses to let her friend die, vowing to protect her from a growing bloodlust.