At the elite boarding school Myoujou Academy, there exists the special "Class Black." Disguised as a normal class, it serves one purpose: assassination. At the center of Class Black, one student is the target while the others are assassins who are all itching to make the kill. The student who successfully assassinates them will have any wish of theirs granted, but they only have one chance to do so; expulsion awaits those who miss their mark.

Azuma Tokaku, an apathetic assassin-in-training, finds herself transferred to the latest Class Black with 12 other girls. One girl sticks out from the rest—Haru Ichinose, a cheerful girl who is idealistically insistent on making friends and graduating. To little surprise, the students discover that Haru is indeed the target and begin to plan their assassination. However, Azuma feels a growing fondness for Haru and is drawn to a different course of action. Now, for the first time in her life, she has found a cause to fight for.