All the four cousins are down with fever and coughs. After a bit of recovery, Aunt Fanny sends them to the mountains for better air for their coughs. Near the chalet they are staying at, they hear strange sounds coming from the big house nearby and see fantastical mists in the air at night. Soon they discover that the house is not empty. In fact, it is housing an old lady that looks rather confused and distraught. The Five decide to investigate all these mysteries. Can they solve all of them?