While cleaning Lille DIPJ station, Morgane Alvaro, housekeeper, with an IQ of 160, lets drop the file of the murder of Antoine Levasseur, found dead at his home. She can't help reading it and finds out that Jeanne Levasseur, his wife, missing since the day of the murder, is innocent. PJ finds out that Morgane has been messing around with the Levasseur files and Commissaire Hazan, being pressed by her superiors, ask her to help them with the investigation and the disappearance of Jeanne Levasseur. Commandant Karadec is then obliged to collaborate with Alvaro.