After solving the Amekoushi no Kan case, Ichiyanagi Nagomu receives an invitation from his friend Hiyori to go to the Ruroy castle in Europe. The building is hidden deep within the valley and the only way to enter is through a rope bridge. Nagomu learns of the history of the castle during dinner with the current owner of the castle. The mystery begins.

The ancestors of the current Master of the castle have been studying black magic and satanic rites. He has been trying to summon a demon by offering youths as sacrifices. After numerous failed attempts, he finally managed to call upon a demon to possess his sacrifice. The castle, is then, built in the valley to keep the possessed youth from escaping. The youth's mother has entered the castle to find her son, but she, too, disappeared not long afterwards.

According to the rumors, the ghost of the mother is still in search of her son and that the youth is still somewhere within the castle.

During the conversation, the castle blacked out, and in the darkness, the occupants can feel the evil in the air.