Series 4 starts in France on Canal Plus next Monday, 3 September - the first two episodes back to back.
Here is the resume of Episode 1 translated approximately (not a spoiler)
Capt. Laure Berthaud and her group the DPJ discovers the corpse of a student mauled by a roadside bomb. The lawyer Josephine Karlsson defends Moussa Koné, who's been an undocumented resident in France for ten years and who is threatened with expulsion. These two cases are found to be linked via a network of citizen protest opposed to immigration policy. For Capt. Berthaud it's a particularly turbulent period: a new divisional commissioner arrives and she is the subject of an investigation
If you want to see the trailer for yourself go to