In Paris, police search for a psychopath whose murders are inspired by Jeanne Deber, known as "The Mantis", a famous serial killer who terrorised the country 20 years ago. Believing it takes one to know one, the police asks Jeanne Deber to help hunt the copycat. Placed in solitary confinement since her arrest, "The Mantis" agrees on one condition: to deal only with Detective Damien Carrot, her estranged son. Damien has no choice, for a serial killer is on the loose and could strike anytime, anywhere in Paris.
With the clock ticking and his family in danger, Damien weighs an offer from the killer. A rescue mission takes an unexpected turn.
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.
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.
Faced with a series of grisly unsolved murders, a police commissioner turns to imprisoned serial killer Jeanne Deber and her estranged son for help.