The time is the middle ages, the place a remote region of Europe, in a small village surrounded by mountains.
There is a village that is ruled by a heretic noble, in an impassable region.

The protagonist is a young nun named Mary who doesn't know her parents, she can't remember her childhood and was raised at the church.
Getting closer to her 16th birthday she starts having nightmares every night.
They seem like memories of a person that is herself, but not herself, she can't remember the contents very well but she can tell they are dreams about that 'other self' doing something frightening.

And so, on the night of a full moon the noble Gerald who lives in a castle at the summit of a mountain comes to the village with a request.
On the day of Mary's 16th birthday he want's her to offer herself up to him in marriage.

From that day, strange things start happening in the village...

[Translated from the official site]