Fumi Nishioka is an apprentice "Sweeper," a role involving the cleansing of negative emotions from human hearts, and she works alongside fellow Sweeper Kyuutarou Horikita and his family. Prior to meeting them, she was homeless, and wherever she went, people treated her as if she were cursed. However, the Horikitas—particularly Kyuutarou—changed that situation and provided her with a place where she truly belongs. Yet, as the Horikitas are shocked to discover, a great but terrible power lies dormant within Fumi's body, which can be wielded for either good or evil. Can Fumi control it, or will it end up controlling her?