A tranquil summer day in the Moomin Valley is interrupted by a volcanic eruption and a huge surge. Water rises higher and higher, but the at the last minute the Moomin family finds refuge in a strange house that floats to their rescue. It turns out this is not an ordinary house but a theatre stage, and the home of certain theatre-loving Emma. Somewhat reluctantly Emma agrees to help the Moomin family who want to produce a play which has been written by Moominpappa.