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.
Faced with a series of grisly unsolved murders, a police commissioner turns to imprisoned serial killer Jeanne Deber and her estranged son for help.
As a furious Damien tries to figure out what his mother is up to, a clue found on Séverin's computer leads the team to the next potential victim.
Damien uncovers a secret that sends him racing back to his childhood home. Unable to shake her suspicions, Lucie digs into her husband's past.
The Mantis makes her next move, Achille finds a crucial clue linking the victims, and Damien pays a visit to his mother's psychiatrist.
When Jeanne receives a live video stream of the next victim, the team scrambles to stop a murder. New clues to the killer's identity emerge.
With the clock ticking and his family in danger, Damien weighs an offer from the killer. A rescue mission takes an unexpected turn.