A skilled assassin named Tokaku Azuma is transferred into Myōjō Academy's "Class Black", tasked with identifying and killing a target amongst her twelve classmates, the rest of whom are also assassins. At her first homeroom class, consisting of five of the total 13 students, Tokaku finds herself drawn towards a kindly girl named Haru Ichinose, believing she may be the target. As Tokaku spends more time with Haru, who is revealed to be her roommate, she notices some knife wounds on her legs. After encountering her various classmates throughout the evening, Tokaku, who had also been tasked by her teacher, Kaiba, with solving a puzzle reading "The world is filled with (___)", becomes bewildered when Haru suggests "forgiveness" as an answer.