Beast ~Injuu no Yakata~

The main figure of the game is Muneto. He and his 4 friends are caught in a rain under a forest exploration. Timely they see a light from a house. It's a mansion of question. They come to it and beg allowance of their stayover. A servant Sandaihu rejects it but fortunately and kindly the mistress Kiriko accepts it. Thus Muneto and his friends have entered the land of beast.
They sleep in each room. Muneto spends night playing H with his beloved Kotone and later Kiriko. Now, as Muneto wakes up in next morning, people have disappeared except Sandaihu. What's going on? From now on you must clear the game to save friends.

[From Anime Densetsu]