Mountaineer Bernard Lang's body is discovered with a broken neck in climbing gear at the foot of the Luxor Obelisk in the Place de la Concorde. Fascinated by the obelisk and its history, Jo knows that the monument is also a great vantage point of the surrounding area, including the suites at the Hôtel de Crillon. And in one of them, that evening, a well-bred young woman (Miranda Raison) was cheating on her husband.

Charlotte, a 35-year-old Mirage pilot, is found dead in front of Les Invalides, wearing a ring stolen a year earlier during the murder of a family in the south of France. Her lover, a mysterious mechanic, is quickly suspected, but while unearthing the details of that forgotten crime, Jo discovers a conspiracy.

In a parking lot on Place Vendôme, firefighters uncover the charred body of Philip Roquin, an accountant for one of the square's many jewelry stores. His wife admits that he was kidnapped and that a ransom was requested. The investigation leads Jo Saint-Clair to a prison where a woman called Lisette was incarcerated.

Marie-Eve Lambert, a young gallerist, is found lying on the Place des Vosges, killed by a violent blow to the head. Believing at first in a crime of passion, Jo discovers a mysterious message in the victim's hand which leads him to the Archives Nationales, where Marie-Eve was doing research on women in the French Resistance deported to Auschwitz.

Leaving the Palais Garnier where he had just dropped off his daughter for her dance class, Raymond Sittler is assaulted on the steps of the Opera. A few seconds later, he starts convulsing, collapses on the porch and dies. Initially suspected, his mistress is also murdered at the Barbès-Rochechouart station, through poison. The investigation leads Jo to the prestigious Sorbonne University and to the door of a troubled student and his ambitious girlfriend.

A young geneticist was stabbed in the Paris Catacombs, a Satanist symbol etched on her back. Jo is convinced that this is a crime of passion. But the case takes an unexpected turn when the team discovers that the victim was conducting experiments with plague bacteria recovered from skeletons of the catacombs. The trail leads to a mysterious woman who could well be a serial killer on the run. She seems to already have chosen her next victims: the family of a prominent politician. The killer turns out to be Nicole Wallace (played by Olivia d'Abo), the one-time nemesis of Det. Robert Goren in the series Law & Order: Criminal Intent (also created by René Balcer).