Fern King is about to uncover a place that she could not have imagined in all her wildest dreams. Annwn is the dream mirror of our world, a place where Dreamers walk in their slumber, their dreams playing out all around them. An enchanted, mysterious place that feeds our own world — as without dreams, without a place where our imaginations and minds can be nourished, what kind of humans would we be?

But Annwn is a place as full of dangers as it is wonders: it is a place where dreams can kill you. Annwn and its Dreamers are protected by an ancient order known as the Knights — and when Fern's hated twin Ollie is chosen to join their ranks, Fern will have to do whatever she can to prove she is one of them too.

But the world Fern discovers in Annwn, in this dream mirror of her London, is a fragile one, threatened by vicious nightmares. Nightmares that are harder and harder for the Knights to defeat. Something dark is jeopardising the peace and stability of Annwn, something that must be rooted out at all costs. And gradually, Fern realises that the danger lurking inside our sleep is more insidious and terrifying than any nightmare. Because if you can influence someone's dreams, you can control their thoughts ...