I wrote a couple of years ago about the 2nd series of this 'gritty' French television 'cop' drama.
Since then I've seen the 3rd series, equally-astonishing in its 'grittiness', but just recently I've at last seen the 1st series, which I missed the first time around. This evening, in fact, I've just watched the final two of the eight episodes of this marvellous drama, like all the three series it was shown on BBC4 television. The adjective 'gritty' doesn't quite do it justice though - perhaps 'stomach-churning', 'hurl-worthy' or plain 'gruesome' would sum it up better!
It would be wrong and insensitive to say I have grown to 'know' and/or 'love' any of the characters in this bleak drama, but I suspect it is, whilst no doubt over-dramatised for the purposes of producing a television series, unfortunately not too far removed from the harsh daily reality of many involved in some of the nastier real-life crimes I've heard about in recent years.
I do hope a 4th series is in prospect? - and from this website of French producing company Canal+ it seems that filming of the 4th series began in August this year and will be completed by next April, with the series being expanded from the usual 8 episodes to 12 for this 4th series. What new horrors they will have in store for us can only be imagined! You can find a rough translation into English of the synopsis of the 4th series, linked to above, here.