Jonathan,11, spends most of his vacations alone with his grandfather, who lives on the outskirts of a big forest teeming with mythological creatures. This year, he is joined by his younger sister, Sophie. However, he does everything he can to prevent being trapped with a younger sister. He succeeds far too well: Sophie is abducted by a 1000-year-old colossal bear. Jonathan is now forced to journey into the heart of the forest in order to confront the weird beings that live there and rescue his sister.