After a year of working at home in order to control his synaesthesia (the rare neurological condition that causes his brain to crosswire his senses), DCI Mark Lapslie gets a visit from Sergeant Emma Bradbury, summoning him back to active duty. A television reporter has been brutally murdered and the top brass think Lapslie is the man to investigate the crime. With no witnesses and no suspects, Lapslie finds himself facing one of the toughest cases of his career. Then he has a second murder on his hands: a man is found dead in a train-station bomb explosion. Lapslie begins to suspect that the two high-profile cases may be connected. Under pressure to produce results and with the media bearing down on him and his synaesthesia going wild, Lapslie approaches his breaking point, but the solution to what's beginning to look like the crimes of an especially evil and diabolically creative serial killer remains elusive.