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.

After making a gruesome discovery in the walls of subterranean Paris, Catherine and Julie's partner, Raphael, dig into Julie's past for clues. The police find a lead on Julie's whereabouts, but Catherine has a different hunch.

Catherine and Raphael track down a lead and obtain critical information on why Julie disappeared. As they connect the dots between Julie's case and what they found in the catacombs, another threat is discovered.

Pretending to be dead, Catherine gains access to an underground lab where she meets a man who may be the last person to have seen Julie alive. Catherine and Raphael learn that he may have put her life in even more danger.

More key players involved in Julie's disappearance are revealed, and they're determined to keep the truth covered up. Meanwhile, Catherine and Raphael enlist an unlikely ally in their search, who helps lead them to a disturbing discovery.

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.