At Myojo Academy, an elite all-girls boarding school, the Class Black is comprised of thirteen students, twelve of whom are assassins. Their prey is the thirteenth student, Haru Ichinose, an upbeat girl who vows to survive school and graduate unharmed! Enter Tokaku Azuma, a cold-blooded killer who transfers from another school to kill Haru. What should have been a routine hit turns into a far more complicated affair as Tokaku is unable to pull the trigger on Haru. As Tokaku copes with her strange, budding feelings, will she decide to protect Haru's life from the other assassins or to end it?