An underwater volcano off the coast threatens Moominvalley. A huge cloud of smoke signals that the volcano is active again, and shortly afterwards a tidal wave sweeps over the land. Only the top floor of the Moomin house is visible above the water, and the Moomin family, Sniff, Snorkmaiden, and Little My save themselves by climbing onto a house that is floating by. As the Moomins explore their floating home, they discover that it is a theater.