As the Moomins prepare to hibernate through what is going to be the worst winter yet, several unwelcomed guests take advantage of the Moomins' generosity and keep the family awake throughout the long winter. Their quirky but needy guests prevent the Moomins from hibernating and the chaos only increases with the arrival of a little nibling determined to find out everyone's secrets. One by one, the nibling sees what the Moomins and each of their houseguests do when no one else is looking. But everyone is ashamed of what the nibling has seen and is determined to keep their secret activities, well, a secret!