Young police lieutenant Julie Susini goes missing during an underground operation. Her mother Catherine comes out of retirement in search for her.

The police investigate the mass grave; Catherine tries to find out out why Julie wanted to resign from her position.

Catherine and Raphael discover Julie was being stalked by someone; a new grave is found containing recently deceased corpses.

Raphael and Catherine attempt to get in touch with the gravedigger; Catherine also seeks her ex out for help.

Garraud and a colleague set out to find the identity of the gravedigger, who is being held by Catherine and Raphael; the gravedigger tells them the truth.

Catherine tries to infiltrate the Nox site on the dark web with a hacker; a fierce struggle ensues in the depths of Paris.