Commissioner Rousseau (Patrick Chesnais), from the criminal division, and his young assistant Mariella de Luca (Camille Panonacle), lead the investigation of a case of a head found in the Paris catacombs.
The victim, Maximilien Müller, was a well known contemporary artist with a troubled past. In the artist's workshop Rousseau and de Luca discover another corpse that has been decapitated. They are unable to find her head.
Eventually it is apparent there is a serial killer on the loose. The killer is seeking revenge for an injustice committed a long time ago.
The key to finding the killer is the widow of the artist, Lydia Müller (Ludmila Mikaël). She is also very possibly herself in danger from this killer.