In short, it's your typical urban police procedural murder mystery. People have been murdered, and it's down to two competing police departments (the local police, and Norway's equivalent of the FBI) to find the killer.
The killer is caught in the end, but it's a depressing book. I feel this because there was no real uplift at the end. Usually after reading a mystery, there's a sense of relief that the baddie was caught. I mean sure he was caught, but there was no sense of "phew" afterwards. Because of that I felt the book was depressing. Don't get me wrong, it was a good read, just depressing all the same.