The Butterfly of the Night (夜の蝶, Yoru no chō) is the 6th episode of Muhyo & Roji's Bureau of Supernatural Investigation anime series.
Synopsis
Muhyo, Roji, and Nana visit a rustic hot spring resort to meet a client. They bathe in the hot springs while waiting for him... and then Roji catches a middle-aged man peeping into the women's bath at Nana. The man turns out to be their client and the author of a current best-selling novel series, "The Butterfly of the Night," Yontani Abeyuki.