In a classroom where students are being killed in grotesque and impossible ways—beheaded by a flying ski, a subway door, or a crashing traffic light—a beautiful transfer student named Tatejima Hitode arrives, clad in a jet-black sailor suit. The deaths are no accident: the entire class is under a curse, and the only way to survive is to sacrifice one of their own. Hitode, enigmatic and unreadable, seems to hold the key to the curse, but her true allegiance remains murky. She calls herself "Hitode-chan," a pun on "hitodenashi" (inhuman), and her presence stirs a volatile mix of envy, fear, love, and hatred among the students.

As the body count rises, the story unfolds through multiple perspectives, shifting focus from one student to another. Key characters include those who try to understand the curse and those desperate enough to exploit it. The first arc centers on a boy who sought revenge on a bully by invoking a dark power, only to unleash the curse on the entire class. Later arcs peel back layers of mystery: the identity of a black-clad woman who shadows Hitode, the true nature of the curse, and the question of whether Hitode is friend or foe. The only way to break the chain is for someone to willingly become the sacrifice, forcing the survivors into a brutal moral dilemma. The narrative blends supernatural horror with psychological suspense, exploring how ordinary people behave when pushed to the edge.