During a police operation taking place in Paris' underground tunnels, young lieutenant Julie Susini disappears under the eyes of her partner, Raphaël Berger. The police search party find nothing.

The police interrupt the underground search as Julie's passport has been scanned in Spain. Catherine and Raphaël think it's just a cover-up. They investigate the murky areas of Julie's past: she was troubled, wanted to quit her job, and even made contact with the IGPN.

Catherine and Raphaël find out that a policeman was tracking Julie. Under threat, he reveals why she disappeared.

Before the police arrive, Raphaël and Catherine force the gravedigger to talk: He handed her over to a mysterious killer called 'Nox'.

As police chief Garraud inspects the underground lab with one of Julie's colleagues, we find out that they too are involved in her disappearance.

Catherine pays a fortune for her and is led by Nox administrators to the underground room where Julie is held. So, the final act of this story unfolds, a desperate fight for survival in the darkness beneath the city.