Thaniel, just seventeen, is a wych-hunter. Together, he and Cathaline - his friend and mentor - track down the fearful creatures that lurk in the Old Quarter of London. It is on one of these hunts that he first encounters Alaizabel Cray. Alaizabel is half-crazed, lovely, and possessed. Whatever dreadful entity has entered her soul has turned her into a strange and unearthly magnet-attracting evil and drawing horrors from every dark corner. Cathaline and Thaniel must discover its cause-and defend humanity at all costs.