More than a year has passed since Moss was released from the clutches of the Riverwitch. Now her father has swapped his bloody axe for a blacksmith's forge and they have moved away from London, taking Salter with them. But strange things are happening on the river and the Riverwitch is lurking again. Moss has no choice but to leave her new home on a deadly journey to put an end to the evil that is enveloping the Tower like a stinking fog. It's a decision that may cost her her friendship with Salter and ultimately her life.