In Hervanta, the Tampere police's nightmare comes true: a 16-year-old girl is found strangled with fishing line in a forest. It seems that a mentally ill murderer is on the loose, but the police have no idea who he is. The father of the murdered girl goes into a vengeful rage and launches his own investigation.

The Fishing Line Strangler has struck again. Koskinen and his team race against time to find the killer as the number of victims grows. A former colleague, Roine, who has ended up in a psychiatric hospital, provides a crucial clue. Will Koskinen have time to act before an innocent man is murdered in revenge?