On her 16th birthday, Himari Momochi learns that she has inherited a mansion named "Momochi House," the last tangible link to her real family after growing up in an orphanage. Despite rumors of haunting, she dismisses the ghostly tales and, upon entering the mansion, encounters Aoi Nanamori and his companions - Ise and Yukari - who seem to be occupying her newfound residence. Undeterred by their warnings to leave, Himari is determined to be the one to make them depart.
Her resolve is swiftly tested when she is attacked by youkai, discovering that the house exists in the liminal space between the world of the living and the spirit realm. To her surprise, Aoi was chosen as the guardian of the house, a role initially intended for Himari. However, this formidable power comes at a steep cost - Aoi is bound to the mansion, his existence erased from the human world. Ignoring his pleas to vacate the premises, Himari pledges to assist Aoi in breaking the curse that confines him to the house, committing to share the same roof with him in their quest.