The members of SOX receive an invitation to the Nukumi Resort allegedly from Sophia Nishikinomiya. Tanukichi remembers how his father had given him a valve handle for use if he should ever go to Nukumi. This makes them decide to go there despite their misgivings. At the resort they see Gouriki and Anna, who have received similar invitations.
They go to the baths, where Anna has entered the men`s side in an attempt to seduce Tanukichi, but as she does this, they hear Sophia just outside being attacked by someone. They see her at the entrance to the baths, unconscious, and dressed in black underwear. She too received an invitation to go to Nikumi, allegedly from Anna, which she claims she never sent. They meet the one who sent the invitations, a man covered in black panties. He has taken all of their clothes and left them black underwear to wear instead.
The man in black explains how the valve handle Tanukichi has is the key to a great treasure, They manage to escape, recovering the valve handle, and explore the ruined buildings to find a place where it will fit. They find a valve stem that fits it exactly, and turning it, they find a treasure trove of erotic art that had been hidden from the censorship state. They seal it back up again, hoping that it can be opened in the future when the country is ready for it.