Two Cistercian monks are brutally murdered near Lille; one of them was an old friend of commissar Niemans. Together with Camille Niemans engages in the investigation of the homicide, led by Sabrina Harel. In the monastery of the monks, they come upon evidence that suggests that the two victims, brother Guillaume and brother Jean, in their research on the origin of the musical notation, apparently made a discovery that could be related to their murder.
When the mayor's daughter disappears, Camille is convinced that the rock singer Messionaire is behind it, whom she had jailed eight years ago for rape and other serious violent crimes. She knows that Messionaire is on the loose again and is on tour with his band BHP at the time of their investigation in the vicinity of Lille. When Niemans mentions that his friend Jean-Louis alias Brother Guillaume had brought out the words sword and snake shortly before his death, she is sure that the two cases are related. When visiting a concert by BHP, she noticed that Messionaire had tattooed exactly these symbols on his forearm.
In their search for the mayor's daughter, the police find the girl dead in the dunes, next to a plaintive monk from the Cistercian monastery.
