Eleven-year-old Jeff and his father live in a camp next to a dilapidated old castle owned by their family. The two of them have been all alone ever since Jeff’s mother mysteriously died seven years ago. But summer holidays are here and Jeff can enjoy the company of his friends who have come to the camp with their parents. When a dire financial situation forces his father to consider selling the castle, Jeff and his friends embark on a quest to find the lost treasure from the legend of the water fairy Melusina. In the race against time, the little argonauts will encounter many adventures and challenges.