Makishima Tantei Jimusho Monogatari ~Shinsou no Malice~

Early Showa era... A Western-style mansion stands alone in a remote rural area. Every night, pleasurable affairs happen here. One night, something strange happens. A letter of request arrives to the protagonist, who runs a detective agency in the imperial capital of Tokyo. "My life is in danger, and I want you to uncover a conspiracy..."