A tranquil summer day in the Moomin Valley is interrupted by a volcanic eruption and a huge flood. Water rises, but the Moomin family finds refuge in a strange house that floats to their rescue. This is a theatre stage, and the home of a certain theatre-loving creature named Emma. Reluctantly Emma agrees to help the Moomin family who want to produce a play to pass the time. Problems arise when Moomintroll and the Snork Maiden are separated from the others by accident. During their journey back to the theatre they meet Fillyjonk and spend a wild Midsummer Eve around a bonfire. However, a misunderstanding the following morning makes them fugitives chased by the authorities. At the same time Little My is also separated from the rest of the family. She runs into Snufkin who has been wandering on his own. The pair of them challenge the autocratic Park Keeper and end up with a flock of orphan children to look after. The inhabitants of the Moomin Valley have been thrown about by misadventures.