It is ending August, winter and cold will come soon. Papa Mumin has gone in search of the perfect place to build his new home. Mama Mumin and young Mumintroll begin to worry about him, because he has been away for many days and they have no news of him. Thus they begin a search that will take them to the depths of the great forest, to fantastic landscapes, where they will meet extraordinary beings who will help them on their journey. In it they will have to survive the dangers of days and days of rain.