It is Midsummer Night - no time for dreaming. Because sometimes, when there is more than one reality at play, too much dreaming can make the walls between them come tumbling down.
Unfortunately there is usually a very good reason for there being walls between them in the first place - to keep things out. Things that want to make mischief and play havoc with the natural order.
Granny Weatherwax and her tiny coven of witches are up against real elves. And they are spectacularly nasty creatures. Even in a world of dwarves, wizards, trolls, Morris dancers- and the odd orang-utan - this is going to cause trouble….