While on a school trip, high school student Satou Takekiyo found something unusual in the mountains, something his ancestor Satou Norikiyo (known also as Saigyo) left behind over 800 years ago. It was a creature who looked like a girl, but was really a person that had been constructed by Saigyo and then abandoned. She had eyes that shone like an animal's, long black hair, pale white skin, and she was living in a cave surrounded by bones.

The girl followed Takekiyo when he tried to leave her, seemingly convinced that he had come to finally take her from the mountain. Now Takekiyo and his friends must deal with this unexpected and unnatural new addition to their lives, just as she must learn to live in the modern human world!