Olivier and his younger sister, Magalie, go on vacation to their grandparents' chalet. When he arrived, Olivier saw a note on the front door. Warning! Entry prohibited in the Lac des ÒIles marsh. A mysterious creature has taken up residence there… In order not to frighten his sister, the young boy decides to hide the message from her. But their excursion the next day in the swamp has many surprises in store for them.