Nox

During a police operation in Paris’s catacombs, young lieutenant Julie Susini disappears before her partner’s eyes. When the police can’t find her, Julie’s estranged mother, retired cop Catherine, decides to lead her own investigation.

The corpses are mostly illegal immigrants who have been murdered and expertly cleaned. 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.

Catherine and Raphaël find out that a policeman was tracking Julie. Under threat, he reveals why she disappeared: with a group of colleagues she took part in a police self-administered justice killing. They shot three teenagers.

Catherine, faking to be dead, gets into the secret box used to transfer corpses to the « gravedigger ». The box is actually a passageway. Catherine tumbles down into the gravedigger's underground lab.

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. Now that the gravedigger has been discovered, Garraud embarks on a killing spree to silence every witness.

Now that she knows the truth about what happened to Julie, Catherine goes to extreme measures to bring her investigation to a close. But in order to do that, she’ll need to face a desperate fight for survival in the darkness beneath Paris.