Nocturnal Illusions' genre is a mixture of a dating sim and visual novel. In this game the main character decides to get away from it all during his spring break and stay in the mountains. While staying in the mountains a massive storm causes the main character to faint. A mysterious woman saves him and uses sexual pleasure to keep him warm. It turns out that he is now in a strange old mansion. The woman who saved the main character is the mistress in charge of the place. The main character decides to explore and while there meets many women as the story unfolds...

[Edited from Wikipedia]