For Londoner Fern King, Annwn is her second home - the dream mirror of London, the city she loves. An astonishing world, a world where Dreamers walk in their slumber, their dreams playing out all around them. And Fern, along with her twin brother Ollie, is now a Knight, a trusted guardian and protector of those Dreamers - and every night is spent in Annwn, fulfilling that previously only-imagined destiny to guard those who sleep.

But Annwn, this dark and golden city, is a fragile place. There are those who seek to control and to ruin it - mistrust and secrets are rife, and as the delicate symbiotic balance between Annwn and our own world begins to topple, rips begin to appear between the fabric of the worlds. And Fern and Ollie will have to do all in their power to protect this place that has become so precious to them ...