A rash of mysterious thefts hits the school, and rumor has it that the apparition "Yousei-san" is behind it. When Nene catches sight of the creature, will she end up losing more than just her school supplies?

Nene’s best friend vanishes without a trace and no one else remembers she ever existed! To get her friend back, Nene is forced to call on Hanako for help and journey into a "Boundary" between the world of the living and the world of the dead.

In search of Hanako’s secrets, Nene and Kou enter the fifth Wonder, the 4 O’clock Library. Its books contain the past and future of every student their school has ever had, but some knowledge comes at a price.

Kou gets more than he bargained for when he tries to help a student’s ghost rest in peace. The late Mitsuba may look delicate, but his language is anything but. Can Kou put up with him long enough to help him?

Nene made it back from the Hell of Mirrors in one piece, but she worries about what Hanako may be hiding from her. Just then, strange fish appear, calling her their princess and asking her to join them as the mermaid’s successor.