Louis is a twelve year old boy who lives with his parents and older brother in southern Morocco. Louis is very happy until the day his father shares a secret with him.

In Paris, many citizens go to the precinct after the doors of their apartments have been sprayed with a 4 and the letters "clt". When a dweller is found mysteriously dead in his apartment, Detective Jean-Baptiste Adamsberg and his partner Danglard investigate the case and discover that plague may have killed the victim. Meanwhile, in the center of Paris, the former actor Joss Le Guern survives reading advertisements in a square for the public; when he receives weird messages about an outbreak of plague that is coming to Paris, the former professor Hervé Decambrais requests the warnings and goes to the library to research the meaning of the text, where he meets Adamsberg. Together they find that a maniac is killing people using flees contaminated by rats and spreading the disease in the city; without any clue, the police force do not have how to avoid the panic in Paris.