Negative energy occasionally takes root in human hearts. This negativity can start to form into monstrous creatures, which can only be handled by "Sweepers," special individuals with the power to banish them and restore peace. Kyuutarou Horikita hails from a long lineage of Sweepers, yet most of his classmates view the second-year student at Kurokado High School as a grumpy boy obsessed with cleaning. While in the old school building, he encounters 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.