Though nominally an Inspector Erlendur novel the detective in this book is his team member, Sigurdur Óli. In Reykjavík, Iceland, Detective Sigurdur Óli is approached for a favor by his friend Patrekur. It seems that Patrekur's sister-in-law and her husband have engaged in a spot of wife swapping and are being blackmailed by another swinger couple, Lina and Ebbi. Patrekur asks Sigurdar Óli to pressure the blackmailers to back off and to get their incriminating photos.
Sigurdur Óli goes to the couple's house at the exact moment Lina is being viciously attacked with a baseball bat and fatally injured. The attacker runs past Sigurdur Óli and escapes. Despite his personal involvement in the case Sigurdur Óli joins the investigative team. He soon discovers that Lina and Ebbi owe a large amount of money and frequently engage in extramarital trysts. Thus the detective team looks for suspects among Reykjavík's debt collectors as well as men who have received sexual favors from Lina.
Meanwhile Sigurdur Óli is repeatedly contacted by an elderly alcoholic tramp, Andrés, who has has taken an old man hostage and tied him up in a basement. Andrés is incoherent, however, and can't make Sigurdur Óli understand his situation.
Sigurdur Óli has to dig through layers of intrigue to discover who attacked Lina and why. He also looks into Andrés difficulties and uncovers some shocking secrets.
The book has an array of interesting characters, including Sigurdar Óli's girlfriend and mother, fellow detectives, local thugs, and shady bankers. The story has an engaging plot that leads to a satisfying resolution. Very good mystery.