Lucien, Violette, and their friends go on a school trip to an old castle rising in an magnificent natural setting. The children take educational workshops on biodiversity, local history, and its legends. The woman who owns the castle is none other than the descendant of a line of witches. The children quickly realize that the castle is under a spell: a presence roams it at night in the form of a strange young woman with pale skin. The boys and girls decide to investigate, but are soon split into two groups, each with different methods and goals. Which will be the first to solve this latest mystery?