After solving the Amagoushi no Yakata case, Ichiyanagi Nagomu receives an invitation from his friend Hiori to go to the Ruroi castle in Europe. The construction is hidden deep in the valley and the only way to enter is through a rope bridge. Nagomu learned of the history of the castle during dinner with the current owner of the castle.
The ancestor of the young nobleman has 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 the youth is still somewhere within the castle.
During the conversation, the castle is blacked out, and in the darkness the occupants can feel the evil in the air.
