One morning, Marion is taken hostage by Joël Mège, a young inmate who demands to speak to her. Among the warders, he has recognised one who murdered a fellow-prisoner. This ordeal and Mège's revelations open past wounds for Marion.

While preparing the release of a pregnant inmate who refuses to name the father of her child, Marion investigates Mège's claims and learns that his former cellmate had a contract out on a policeman. The death of her partner Lafaille was therefore planned.

Marion is preparing the release of Henri Poletti, an introvert. At the same time, prisoner Manuel Campo proves to her that Mège was killed by warder Vidal at the hospital. Marion asks her husband for the files on the Lafaille case and asks the replacement, Costa, to investigate Vidal..

Marion is faced with a clash between two gangs: that of Madeleine Liancourt and that of Samia Boukhri. At the Criminal Investigation Department, Armelle, an ambitious cop, sheds new light on the Lafaille case. She discovers that Marion's husband is not as innocent as everyone believes...

Marion asks Vincent why he was present at the shooting in which she almost died. He explains that he was trying to avoid revealing that Lafaille was corrupt. Marion deals with two of his protégés, Alice, an ex-surgeon and Kevin, who is being blackmailed by former partners.

Feeling threatened, Vérac, the Criminal Investigation Department's manager, organises a hit with a killer imprisoned at the penitentiary in order to eliminate Marion. Armelle informs the law of Vérac's corruption. He and Vincent are arrested... Marion has to deal with the death of an inmate following a heroin overdose.