Ten years ago, Essi's mother died in a questionable accident. She left a USB flash drive with a locked folder called "X-note". Unable to understand the purpose of it, Essi continued living her life as a normal junior high school student. However, Essi is hardly normal, for she possesses power beyond normal human.
Essi's quiet life was interfered by a sudden visit of a young man named Yuon. He invited her to investigate a murder case and a mysterious disappearance in Xen Institute, a school that had a deep relation with Essi's mother. Tempted by the possibility of unlocking "X-note" and solving her mother's mysterious death, Essi decided to take his request. But what awaits her there is much more than what she could possibly imagine…