牧島探偵事務所物語 ～深窓のマリス～

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..."