1861. When the body of an unknown woman is found in an Edinburgh close, Detective Constable Faro assumes the killing is a random act of violence - until he finds a playing card, the nine of diamonds, underneath her corpse. His superiors scoff at his suspicions of a serial killer, but days later a man is attacked in the street, and left badly bruised and battered with the nine of diamonds in his pocket. Faro believes there's a connection.

He must contend with other problems, though, if he is to solve the case. Detective Sergeant Gosse does his best to frame suspect after suspect, but remains constantly irritated by his detective constable. And although Faro's sweetheart Lizzie loves him deeply, he is not sure if he feels the same way. And what of Inga St Ola, Faro's first and only true love from his native Orkney? Amongst all this, a servant at Lizzie's place of work goes missing. Could her disappearance be linked to the playing-card killer? Beset by hostile superiors and a police-hating public, Faro feels he may never crack this confounding case.