Natsume, Kitamoto and Nishimura take a trip to an old inn to spend some quality time doing their homework. Surrounding the inn is a lake; that is believed to be home to a mermaid.

Natsume gets a call from his old "friend" Shibata, who he once helped with a youkai problem. Shibata wants him to come visit, and Natsume reluctantly agrees. He and Tanuma pay Shibata a visit, and have a pleasant chat at a park. But when Shibata mentions a local haunted house, Natsume finds his visit has turned into more than he expected...