The last members of the secret society known as the Oversight still patrol the border between the natural and supernatural, holding a candle to the darkness. But the society's new members are unproven, its veterans weary and battle-scarred. Their vulnerability brings new enemies to London, and surprising new allies from across the sea.

But most surprising of all are revelations about the Oversight's past, secrets that will expose the true peril of the world in which their friends Sharp and Sara are trapped - the riddle of the Black Mirrors, and what lies beyond. And the catastrophic danger that will follow the duo home, if they ever manage to return.

The dark waters rise. The candle is guttering. But the light still remains. For now . . .