The Moomins' house has collapsed because the ten children that Mula's mother brought with her have been running riot. There is no peace and quiet to be found anywhere, and Snufkin convinces Moomin to build his own house. But Moomin soon realizes that building a house is easier said than done. Everything that can go wrong does go wrong, and in the end, the ten whirlwinds find the spot where Moomin is building his new house.