On the road to the old manor of grandmother Mimi, where his family always gathers at the end of the year, Max Plume discovers a strange footprint in the snow. It can't possibly be of a human, but also not of an animal. According to his grandmother it must be the footprint of a mountain troll, but Mimi often says strange things like that. Max starts his own investigation and dares to, despite the warnings of his parents, venture onto the frozen lake.