The investigation into the disappearance of the Mayor's daughter advances with the discovery of her body in the middle of a bizarrely staged location. While Bertrand loses control and seeks vengeance, Laurène finally gets close to the invisible enemy that she's been tracking for many years.

Laurène cuts her sick leave short to investigate the murder of a quarry worker. In flashbacks to 57 B.C., a Roman expedition takes a harrowing turn.

While Teddy Bear combs through the latest victim's Super 8 footage, Laurène returns to the library to research ancient Celtic rituals.