McCaleb, now married to Graciela and with a baby daughter, is persuaded to take a case from the LA County Sheriff's office. He is asked to profile the killer of an unsolved murder. The victim was a scuzzball who six years earlier had been arrested by Harry Bosch for murder but then released uncharged by the DA's office.

In doing what he does best - reviewing the crime scene tapes and investigative records - McCaleb picks up a clue the sheriffs missed, a plastic owl left at the crime scene that the sheriffs thought belonged to the victim. McCaleb discovers that it was left by the killer as a message. He traces this symbol to an icon used as a sign of evil in renaissance paintings, and finds that one of the most frequent users of this iconography was none other than Hieronymus Bosch, which makes Harry a suspect. The book becomes an examination of Harry Bosch through the eyes of McCaleb.

Meanwhile Bosch is in the midst of a high stakes trial being covered in the press by Jack McEvoy. Bosch was lead investigator on a murder case that saw the arrest of the son of a wealthy and powerful man. As the trial progresses the defence strategy becomes clear: put Bosch on trial instead of the rich kid, make the focus on Bosch and his methods. McCaleb and Bosch, first at odds, must now work together to clear Harry's name...