In this new adventure in the series, a tsunami floods the Mumin Valley on the night of St. John, and the homeless and homeless Mumin family will find refuge in a floating theater drifting. At first they think it is a very particular "house", but when they arrive in a distant bay ... they realize that what is expected of them is the performance of a theater performance. They get to work and present the drama The Lion Girls, which is not going to go as well as they hoped ... But, after all, it will have given them the opportunity to do theater and have fun.