Genmukan ~Aiyoku to Ryoujoku no Inzai~

You play as Satoru, a detective that smokes 40 cigarettes a day. He and his lovely lady assistant come upon a mysterious parcel at work with an address and the single request of "Protect this family". When he scouts out the address he finds an enormous estate with an equally breathtaking European-style mansion. He heads to the building to further investigate. There he finds the steward, the maid, the manservant, and even the young-lady of the house along with her tutor. However, the master of the premises is nowhere to be found. He then realizes that he's in too deep, but finds out that he's locked within the pavilion. He will soon witness the wicked perverted secret behind the mansion.

[From a forum post by White Masq]