Paris, the year 2000. Commissioner Adamsberg is intrugued by the four inverse images, painted on the doors inside different apartment blocks in the capital. On square Edgar Quinet, Joss Le Guern gets ready to do his morning cry. For several days, an anonymous client has been sending him mysterious, even incomprehensible messages. Written in 17-century French or even in Latin, these continuous messages send a chill down his spine. What do the obscure messages mean? Should they be taken seriously or are they simply the work of an amateur? Or both?