Jeanne Deber, known as "La Mante", the famous serial murderer who terrorized France more than 20 years ago, is forced by the police to come out of isolation to track down her copycat. She agrees to collaborate on one condition: to have only one interlocutor, Damien Carrot, her son, who became a cop because of the crimes of his mother, and who refuses all contact with her since her arrest.