Mansion of Dreams / Genmukan

 The main character of the hentai, Satoru Kido, works as a private detective in a small town. He takes on a mysterious case involving the disappearance of a certain family heirloom of a wealthy family. Accompanied by his assistant, he goes to the estate where the disappearance occurred. However, Satoru doesn't even suspect that he has become part of a madman's planned performance...