Original French title: "Puzzle."
One of the more complicated and interesting of the "Murder in ..." series. A poison pen email about an unsolved murder in Doulac (a small town very near Saint-Omer) sets off a new series of events, including another murder that draws in the previous lead investigator (Sagamore Stévenin) who ends up reconnecting with a pianist (Elsa Lunghini) left in a wheelchair as a result of a possibly related event.
The scenery is more ... interesting than beautiful. Endless side-by-side canals they call "the marshes." There is of course a romance. There's always a romance. There are a lot of people involved, and I'm not sure I followed everything ... but it was at least more involving than the previous one ("Murder in Provins").