After completing her training in surveillance and countersurveillance, Robin Ellacott receives a woman's severed leg sent by a courier addressed to her at the office. The leg had a scar that reminded Strike to a child from one of his old cases and together with the leg, a note was attached. The note contained the lyrics from the last verse of "Mistress of the Salmon Salt" by the seventies rock band, Blue Oyster Cult.
Strike believes that whoever sent the leg must have a grudge against him. Additionally he believes that they are somebody from his own past. His first suspect is Donald Laing, a former Private of the King's Own Royal Borderers. Laing is a notorious ex-military who is known for his violence and insubordination. Strike rescued Laing's wife, whom he had tied up and tortured and his baby. The child however, died shortly after. Strike's evidence had Laing sentenced for sixteen years' imprisonment. At the time of the delivery of the leg, Laing is already released from prison. Strike suspects that Laing would want retribution for his actions.
Strike's second suspect is Noel Campbell Brockbank, also a former army who served in the first Gulf War. Brockbank married a military widow with two daughters and later had his son born when he was serving in Bosnia. Strike and his former partner Hardacre was investigating a rape case involving Brockbank and were both convinced of his guilt when they went to his house and he launched himself at Strike with a broken beer bottle. Strike managed to punch him until he hit the wall however he still managed to escape the rape charges because he was deemed unfit for trial. Apparently, he was suffering from mental illness stemming from PTSD from his military life. Brockbank is a pedophile and rapist, and Strike suspects that he would hold a grudge against Strike for causing the failure of his marriage and subsequently had his son taken away from him.
Strike's third suspect is Jeff Whittaker, his ex-stepfather who is only a few years older than him. Strike is convinced that his mother's death of heroine overdose was actually a murder committed by Whittaker himself. Whittaker is a rock star wannabe whose interests include the Satanic Bible and getting high. He was a problem child, having been expelled from his boarding school and doing time in a correctional facility. Whittaker was in and out of prison for various offences and Strike suspects him because Whittaker hates him since Strike testified against Whittaker in court. In addition, his mother had a tattoo on her leg with the same song title as the lyrics written in the note sent together with the leg.
As the story progresses, the search for the sender of the leg evolves into the search of a serial killer who mutilates the body of his victims, called The Shacklewell Ripper. Strike and Robin have to battle against time as the longer the killer is on the loose, the more devastation is caused, not only to public safety, but also to Strike's business. His business is dwindling due to potential clients turning away from a private detective that is highly-publicized in the press with the connection to a severed leg. At the same time, a lot of things take place in Strike's and Robin's personal lives as Strike begins a new relationship and Robin's relationship with her fiance Matthew continues to strain over his dislike of her working for Strike.