After leaving Lockwood & Co. at the end of The Hollow Boy, Lucy is a freelance operative, hiring herself out to agencies that value her ever-improving skills. One day she is pleasantly surprised by a visit from Lockwood, who tells her he needs a good Listener for a tough assignment. Penelope Fittes, the leader of the giant Fittes Agency wants them--and only them--to locate and remove the Source for the legendary Brixton Cannibal.