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...

Another corpse, that of an adult, throws the investigation into disarray. Who could this woman be, found in a grave that isn't hers? Upset by his mother Catherine Valant's revelations, Romain struggles to keep his cool and could well be sidelined from the investigation by Noémie... who, for her part, decides to finally face her own demons by agreeing to a dive with Hugo. A surprise visit from the prosecutor could then definitively put an end to Captain Chastain's mission... if it weren't for the unexpected and surprising appearance of someone we never thought we'd see return to Avalone...