It is hotter than ever before in Moominvalley. The river has run dry and everything is wilting. Going down to swim one day with the other children Little My finds a large wooden crate floating in the water and when they open it they find it full of strange-looking, tropical seeds. They plant them as they have nothing better to do and next night the rains finally comes, a veritable deluge. The seeds sprout and next day the house is surrounded by a jungle. Bamboo, orchids, fruit-trees, but also some much more dangerous plants...