Natsume, Kitamoto and Nishimura take a trip to an old inn to spend some time doing their homework. Surrounding the inn there is a lake in which is believed to live a mermaid.

(Source: Wikipedia)

Natsume's old classmate Shibata (from "False Friend" / "Itsuwari no Yūjin" (偽りの友人)) calls. Natsume visit Shibata together with Tanuma, where Shibata tells them about the "Doll Mansion of Sotogi". A youkai from the mansion later attacks them.