Gangs stalk the streets of London in the fourth melancholy police procedural starring Detective Inspector Tom Thorne. Twenty years ago, Jessica Clarke was mistaken for her best friend Alison Kelly, daughter of ganglord Kevin Kelly, and set on fire. Retired DCI Carol Chamberlain worked on the case that got the arsonist responsible, Gordon Rooker, put away. Now, Rooker`s up for parole, and someone on the outside is harassing Carol, claiming to be the true culprit in the Clarke crime. Frightened, Carol asks for help from DI Tom Thorne, currently occupied with investigating a brutal series of gang killings connected to a rapidly heating-up turf war. As he pursues both cases, he discovers the connection between them.