In season 3, Maria Kallio has completed her final exams and graduated from the Faculty of Law. Her life feels balanced and she is at last in a good place after her divorce. Maria heads up the homocide police department, with detectives Koivu and Beaktas on her team. They are investigating the murder of a Polish businessman, and who turns out to be the husband of a Finnish diplomat.

In the middle of the investigation, Maria and Beaktas are transferred to the National Bureau of Investigation to examine international organised crime. Maria quickly realises her mission is to find the mole in the police force who, has been leaking information, allegedly covering the tracks of a Polish criminal league in Finland.  

While Maria is investigating other people's secrets, she must face a tough fact: she has more hidden secrets in her own past than anyone under her investigation.