In Château-Thierry, home of Jean de La Fontaine, the president of the court was found dead on the steps of the courthouse. On his body, the murderer left clues referring to the fable “Les Animaux malades de la peste”. The investigation is entrusted to Captain Laetitia Alfonsi, who is none other than the victim’s illegitimate daughter. When Thibault, the legitimate son, threatens to reveal his parentage, she is forced to team up with this arrogant half-brother. A second murder inspired by the fable precipitates the collaboration of this unexpected duo. Together, brother and sister will learn to team up and perhaps even see each other as family.