In human hearts, negative energy sometimes finds a place to dwell. This negativity can begin to take shape into monstrous creatures, which can only be dealt with by "Sweepers," special people with the ability to banish them and bring peace.

Kyuutarou Horikita comes from a long line of Sweepers, but most of his classmates see the second-year Kurokado High School student as a grumpy boy obsessed with cleaning. While in the old school building, he runs into a sleeping girl named Fumi Nishioka, a new transfer student who, as it turns out, has nowhere to call home.

But when Kyuutarou and his family realize that Fumi can fight against negative infestations, they invite her to stay with them as a live-in housekeeper and begin training her as an apprentice Sweeper. However, just as negative energy can reside in human hearts, a latent power sleeps within Fumi, and a malicious group wants to use it for their own ends.