Kotou no Oni

The story takes place in 1924 in Tokyo. A handosome young trader named Kinnosuke Minoura learns that his fiancé is murdered in a locked room. The elite doctor Michio Moroto, who's been in love with Minoura for a long time, asks to help solve the case. (Source: AnimeNewsNetwork)

*"We will never again emerge onto the surface. In this world, you and I are all humanity."* Early Showa era, Tokyo. A beautiful young man, Minoura, has a lover named Hatsuyo, who he is engaged to marry. One day, Hatsuyo is murdered by an unknown assailant. Minoura speculates that Moroto Michio, who had feelings for him in their school days, may have killed her out of jealousy. Moroto is a perfect man, smart and good looking, but he is only interested in Minoura. For the sake of his beloved, Moroto devotes himself to cooperate with his detective work, but...?