Assigned to a police station in a small town in Occitanie after an operation that disfigured half of her face, Parisian police captain Noémie Chastain sees her forced convalescence mission take a dark turn. While she was supposed to report on the police station's activities and its possible closure, she suddenly finds herself plunged into a missing persons case dating back twenty years. A barrel containing the skeleton of a child rises to the surface of the artificial lake, at the bottom of which lies the ancient submerged village. Despite her superiors, who consider her fragile, the captain takes this case head on. But through this strange investigation, she will see a whole buried part of her own life resurface as well...

During Hugo's latest dive, an extraordinary event occurs in the submerged city. This catastrophe nearly claimed a life. Noémie is far more affected than she admits by this investigation, which brings back a childhood trauma. At the home of Jean Casteran, Cyril's father, Noémie and Romain unearth a diary that belonged to the missing little boy. His childhood words directly incriminate his father and constitute new evidence of his potential guilt. But the man is suddenly nowhere to be found...