Negative energy can find a home in the human heart, taking form as monstrous creatures requiring the aid of "Sweepers" - individuals possessing the ability to banish them and bring tranquility. Kyuutarou Horikita belongs to a lineage of Sweepers, yet his peers view the Kurokado High School second-year as a grumpy student obsessed with cleanliness. While exploring the old school building, he happens upon a sleeping Fumi Nishioka, a transfer student lacking a permanent abode.
Kyuutarou and his kinfolk discover Fumi's talent for combating negative energy and invite her to reside with them, undertaking her education as a Sweeper's apprentice. Despite this, Fumi harbors a latent force coveted by a nefarious faction willing to exploit it for their purposes.