What is the main conflict in The Haunting of Alaizabel Cray by Chris Wooding?

"The Haunting of Alaizabel Cray" takes place in a fictionalized London, probably in the late 1800s. After an attack on London known as the Vernichtung, London is overrun with wych-kin. Thaniel and Cathaline, two wych-hunters, encounter a girl named Alaizabel. She seems virtuous, but she is somehow connected with a plot to destroy the world. As the novel progresses, Thaniel and Alaizabel fall in love, thwart the plot of the Fraternity, and discover the origin of the wych-kin.