A troubled detective returning to the force after a protracted absence, is tasked with developing a cold case team. Unfortunately, along with a disorganised admin assistant, he only gets two applicants - one of them a snitch who wants to leave the police to sell real estate; and the other an officer with addiction issues who is regularly on report.

Peter's secret investigation has leaked to the media, causing havoc both in the police house and in the Justice Department where Peter's ex-wife Ann-Marie works. Jorma and Caijsa are forced to work together as they dig into the old murder case.

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Peter himself is suspected of murder when it emerges that the murder weapon belonged to him. Ann-Marie realizes that the Minister of Justice, who himself was involved in the murder convictions against the serial killer, is trying to influence Peter's investigation in a blameworthy way.

Peter and the Cold case group are forced to release their suspected killer. Ann-Marie understands that her own enemies in the Justice Department now know Peter's secret. Jorma and Caijsa's old quarrels eventually rise to the surface and explode.

Peter decides to try to talk to the Minister of Justice about the old murder cases. At the Justice Department, Ann-Marie discovers that someone has broken into her computer in search of something that could get Peter killed.

The Peter and Kalla fall group leads the largest human chase since the Palme murder. The whole world looks at Sweden and Peter is forced to face harsh criticism when they make no progress in the search for the murderer and the disappeared Minister of Justice.

Peter continues to shadow the suspect despite being forced to take a vacation. Caijsa and Jorma lie to their boss to secretly help Peter. Barbro begins to investigate a mysterious disappearance many years ago in the village of Österbymo.

Peter and Barbro stake out police chief Ludwig Larsson, while Caisja and Jorma try to find out why Conny was looking into old case files.

The interrogation of Larsson does not go smoothly while the unit discovers the blackmail scheme Conny set up was much bigger than they originally thought

The suspected killer has managed to escape. Peter and the others in the Kalla Fall group feverishly try to track down the killer and they now find even more unpleasant evidence that all suggests that the killer is planning something awful.

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Peter makes an important discovery about the blackmail scheme and who might have been involved.