18+ Izumo (2003)

My name is Toma Hikaru. I'm going to Izumo Gakuen High School with Miyuki who is my little sister-in-law, Nanami-chan who was my childhood friend, and Nagisa who thinks of me as a rival. I began to have a strange dream of a beautiful shrine maiden, who is praying something naked in a lake. I wonder who she is; I think I know her... I also see a strange room of Izumo Gakuen in my dream. Feeling uneasy, I decide to sneak into the room, not knowing it is the beginning of the long journey.

(Source: AnimeNfo)