Joss le Guern is a town crier in Paris's 14th arrondissement.
He calls out the local news three times a day to all who will listen.
Over the course of a few days, however, a number of enigmatic and disturbing messages are slipped in to the daily news, and he becomes increasingly alarmed.
Superintendent Adamsberg is visited by an extremely troubled woman who has found strange marks on the door of her building: upside down 4s marked out in black paint.
This, and the appearance of the frightening messages, are exactly the kind of mysteries Adamsberg loves.
In the course of his inquiries he begins to sense a sinister and often grotesque menace. And when a charred corpse is found, Adamsberg knows he's dealing with a particularly serious and chilling case. "Have Mercy on us All" is Fred Vargas's masterpiece so far.
She is exceptional at building mood and tension, and, as Henning Mankell portrays the social realities of contemporary Sweden in his Inspector Wallander mysteries, so Vargas does the same for Paris and France.