Youko Takahashi is a university student who possesses an unusual ability: she can see spirits, youkai, and other supernatural beings that are invisible to ordinary people. Living in her apartment is a zashiki-warashi named Zashiko, a childlike spirit that only Youko can perceive. Though Zashiko cannot speak, the two share a close bond, and Youko often acts as a bridge between the human world and the hidden world of demis—beings that are the source of many occult phenomena.

The story follows Youko as she encounters various oddities: a friend who feels constantly watched, a ghostly phone call known as Merry-san, the legendary Hanako-san of the school toilet, and a hotel in Okayama plagued by a strange haunting. With the help of the physics professor Souma-sensei and the cultural anthropology professor Kami-mura, Youko investigates these incidents, uncovering that behind every ghost story or urban legend there is often a demi—a being with its own desires, fears, and inability to communicate with humans.

Key characters include the energetic zashiki-warashi Zashiko, who can only exist inside Youko's apartment; the rational and slightly eccentric Souma-sensei, who approaches mysteries from a scientific perspective; the blunt but knowledgeable Kami-mura; and Youko's friends Kasumi and Sayori, who occasionally involve her in their own supernatural troubles. Later arcs introduce other demis such as the echo-like Yamabiko and a police unit specializing in demi-related cases.

As the series progresses, Youko learns that her ability is more than a curiosity—it makes her a vital link between humans and the hidden demi world. She helps demis who cannot speak for themselves, resolves conflicts born of misunderstanding, and slowly comes to terms with her own nature as someone who is neither fully ordinary nor fully supernatural. The narrative balances episodic occult mysteries with a growing overarching story about the coexistence of humans and demis, culminating in a finale involving a mirror-demi's plan to make all demis happy.