Kuroneko Kan

The game's protagonist is a young man named Okita. His friend Murakami has recently found a part-time job: housework in the Kuroneko ("Black Cat") mansion, which belongs to a rich and mysterious family. However, since then, Okita hasn't received a word from his friend. Worried about this strange silence, Okita travels to the mansion. There, he finds out that Murakami has left. He decides to stay and investigate the suspicious circumstances. But he is not prepared to the assault of wild eroticism that the mansion's inhabitants will soon unleash on him...

[From Moby Games]