Department Q consists of Danish Detective Carl Mørck and his assistants, Assad and Rose, who work in the dreary basement of police headquarters investigating cold cases. While they are looking into new evidence that's been uncovered relating to the disastrous case that ruined the lives of Carl's previous partners and resulted in him being demoted to Department Q, they're assigned another cold case.

In 1987, a brothel owner named Rita went missing. Even though Carl is hesitant about pursuing the case, Assad and Rose start investigating the woman's strange disappearance. When they discover that several other people vanished without a trace the same weekend as Rita, Carl decides they may be on to something significant. He has no idea that the outcome could be his downfall.

Over 20 years ago, Nete Hermansen set in motion a devious plan of revenge against the people she held responsible for ruining her young life in the 1950s. Most prevalent among them was renowned gynecologist Curt Wad, an arrogant and malicious surgeon who founded a secret organization called The Cause, as well as starting the Purity Party political movement. Now elderly, Dr. Wad has made it his life's work, along with others recruited to The Cause, to purify Denmark's population by forcing abortions and sterilization on girls and women considered abnormal or unacceptable. As Carl and his assistants uncover more revelations about the missing people and their connection to Wad, the more determined the evil doctor is to keep his secrets hidden -- no matter what.

Deftly using flashbacks to Nete's life in 1950s and 1987 Denmark, Jussi Adler-Olsen weaves a chilling tale of manipulative evil in THE PURITY OF VENGEANCE. Carl Mørck is a shrewd detective dealing with his share of stress, not only in his personal life, but also with his unusual assistants and others on the police force. This fourth book in the Department Q series is even more compelling and complex than the previous three: THE KEEPER OF LOST CAUSES, THE ABSENT ONE and A CONSPIRACY OF FAITH. I've reviewed all of them and can't wait for the next installment.