A tranquil summer day in the Moomin Valley is interrupted by a volcanic eruption. Luckily the Moomin family finds refuge in a strange, floating house, which turns out to be a theatre. Problems arise when the family gets separated. While trying to find their way back, Little My and Snufkin challenge the autocratic Park Keeper and end up with a flock of orphan children to look after. Meanwhile, Moomintroll and the Snork Maiden spend a wild Midsummer Eve around a bonfire. However, a misunderstanding the following morning makes them fugitives chased by the authorities Through a series of lucky coincidences, everyone makes it to the premiere of the Moomin familys masterpiece The Lions Brides.