Dramatisation of the Swedish crime novels by Arne Dahl, beginning with a two-part thriller in which a string of murders prompts CID Inspector Jenny Hultin to put together a team of detectives to catch the killer before there are more deaths and national panic. Meanwhile one of them, Paul Hjelm, is saved from a disciplinary hearing for shooting a hostage-taker after making it known he was unarmed. Their pursuit of the `Fat Cat Killer' soon exposes tensions within the newly formed group as some of them begin to fear for their lives.