The story starts with high school student Yachi Haruaki receiving a strange, heavy black cube from his father, who is an archaeologist abroad. That night, he hears a strange noise in the kitchen and finds a naked girl with blue hair stealing rice crackers. She calls herself "Fear" and Haruaki realizes she is the cube his father sent. Fear reveals that she is a Cursed Tool, objects once used by humans for evil purposes. Over time, these tools gained human traits but at a cost, including emotions and the ability to harm others uncontrollably. Haruaki tells her he is not afraid of her curse and promises to help her break it. Together with his neighbor Konoha Muramasa and the strict class representative Kirika Ueno, they face dangerous groups and other Cursed Tool challenges.