When a flood sweeps through the valley, the Moomins must find a new house. And with their typical Moomin good luck, one just happens to be floating by. It looks normal enough, but there are curtains where one wall should be, strange rows of lights, and other odd amenities.

Then Moomintroll and the Snork Maiden disappear, and the family realizes that the house may hold the answers to more than they ever dreamed.