Undoubtedly this is a very high quality production, elegantly filmed and with careful details that transport the viewer back by over a century. Unlike a lot of dramas which try to modernise the characters to suit modern attitudes, this stays true to the morality of the era.
There are some surprisingly brutal scenes (not necessarily for the squeamish, which you'll realise from the opening scene), and a couple of slightly suggestive sexual moments.
But scratch beneath the surface and across the first 5 episodes there is no actual story development. I won't include spoilers, but many of the events that happen and which you assume will be important in the future simply aren't followed up. The acting is dry and the episodes are slow. Characters undergo trauma and then appear fully recovered shortly afterwards. Extremely unlikely friendships are formed to enable future events to happen.
The writers clearly knew this was a mess because the sixth (and final) episode includes two scenes where minor characters explain exactly what has happened in simple language. And the ridiculous thing is, half of the characters and locations mentioned in the exposition had barely been included in the previous 5 episodes.
A disappointment.
PS I did not realise this was a "sequel" to Paris Police 1900, and had not seen the original series. Many of the cast are the same, but this story appears to stand on its own. It just isn't a good story.