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.
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.
Receiving a request, Muhyo, Roji and Nana visit Nanagase Inn, but as currently Yontani wasn't there, they are proposed to use the hot spring and they accept. While dipping in the hot spring, Nana sees a peeping tom and Roji manages to capture him. He introduces himself as Yontani Abeyuki, an author and the one that called them. Nana recognizes him as the author of the best selling novel The Butterfly of the Night and is shocked to learn the author of the novel was such old pervert. As she praises his work, Yontani explains he didn't write it, but his other "he". He explains that he have been staying for six months in this inn and each night as he falls asleep, he wakes up and had written a few pages, which ultimately became the Butterfly of the Night. He believes he may be possessed as he has no memories of writing the pages.
Even though they laughed at him, they stayed close to him that night to observe. Muhyo ate too much and went to sleep, while Roji went downstairs to return the foodplates. Nana remained to watch over Yontani who was working on his novel. Downstairs, Roji met Yontani and realized the other one wasn't real. As he went upstairs, he sees only Nana and figures the spirit may be pretending for her and attacks it by creating a talisman with his pen. Nana turns into the spirit and starts to apologize. In the meantime, the real Nana was trying to wake up Muhyo. The spirit apologizes again and introduces himself as Hirata Zansetsu, an author from the Meiji era. Before he was able to write anything worthwhile, he got sick of tuberculosis. He went to Nanagase Inn wanting to relief himself and there met Yuki, a young woman that was also having tuberculosis. They fell in love and he felt he was saved by her, but as the illness took her. He was obsessed with her and wanted to preserve their memories, and decided to write a novel about their love story, but had also died before was able to finish it. Nana realizes that's the Butterfly of the Night story. His desire to finish the novel, bound him to the inn and he waited years to find an author that would be able to finish his story. One day Yontani arrived and seeing him great desire to write something, Hirata decides to pretend to be him and finish his novel. Hirata asks Yontani if he could allow him to finish the series, and Yontani comments that the series isn't sexy enough.
Yontani makes Nana take another bath and tries to force Hirata to look at her, who feels quite ashamed to look at her. Muhyo comes and states that crime is crime and uses Magical Law on Hirata, explaining that what he did could have resulted in someones death. Not wanting to be send in Hell, Hirata transforms and takes Nana as hostage, telling them to leave and let him finish his book or he will kill her. Yontani goes to look after Hirata and finds him in his room, while making Nana write. Nana then gives a manuscript to Yontani and Hirata explains that this is the opening for the final book and wants Yontani to finish it. Yontani explains that he had never experienced love and would not be able to write a romance story. He wasn't able to write a manuscript in years and when he found the pages each night, he decided to claim it as his work, but as it became a hit, he started to feel guilty. As he had called Muhyo, he doesn't feel worthy of writing the rest of the story. Hirata explains that the long years of loneliness and guilt, made him experience what he experienced and that's how he knew Yontani could write his book. The owl that Muhyo summoned, shows up and takes Hirata. Yontani proposes a final line for the book and Hirata finds it wonderful.
Later Yontani was on getting interviewed on TV and the interviewer asks him about the theme of the final book and he states its about the joy of being alive.
- Yontani is the star of another of Nishi's works, Yontani-sensei no Daibouken (Yontani's Big Adventure).