Have Mercy on Us All

In present-day Paris, a mysterious visitor predicts a deadly plague, triggering a tense investigation. Officials confront rising fear as rumors swirl and the case slides from superstition into methodical police work. The ominous prophecy sets the tone for a hunt that will test science against legend.

A mirror-inverted 4 painted on doors becomes a haunting clue that points across the city. Commissioner Adamsberg teams with retired historian Hervé Decambrais to decode the symbol, merging contemporary techniques with arcane history. Their collaboration marks the transition from superstition to careful deduction.

The symbol is explained as a medieval protective charm against disease, not a modern omen. A chorus of omens spreads as a mysterious reader broadcasts messages that Hervé interprets as announcements of a plague epidemic. An archaeologist identifies the accompanying letters CLT, Latin for 'cito longe tarde'—a chilling hint that threads through the case.

When the first corpse appears, black lesions mark the skin and fear surges through the city. The case is quickly labeled the plague, heightening public panic. Adamsberg and his team begin linking the signs to the door-marking mystery.

An envelope from the dead man's apartment arrives flea-infested, spreading tension as investigators trace a vector between fear and infection. Forensic work confirms fleas as a potential vehicle, complicating the simple plague theory. The investigation widens beyond superstition toward a more intricate crime network.

Laboratory analyses push back against a straightforward plague diagnosis, revealing a deadly intervention instead. A vial of Curare poison points to an intentional killing, with victims showing a puncture at the neck. The case shifts from a mysterious epidemic to a calculated murder plot.

Beyond the immediate deaths, investigators uncover a shared biographical thread: the victims once worked for a pharmaceutical company in the Congo. Roubaud recounts how he and colleagues killed the company head to hide their illicit drug business, revealing a deeper conspiracy behind the murders. The connection to the Congo-era past reframes the case as corporate crime rather than a mere public health crisis.

The killer’s familial web begins to surface as François Heller-Devile’s son and his grandmother Clémentine emerge as players in the wider plot. Damas recalls childhood memories of a murder staged as an accident, a memory that haunts him as violence returns. The investigation starts to assemble a portrait of motive rooted in family ties.

Clémentine becomes central to the plot, obsessively fixated on the plague and breeding rats in her basement. She orchestrates the spread of rodent fleas through envelopes to six supposed murderers, turning fear into a weapon. Her actions intensify the danger as the actual killer steps closer to realization.

Damas, troubled by his father’s death, marks doors to spare certain neighbors, signaling a disturbing role in the distribution of danger. The markings hint at a wider scheme that will soon tie back to the family's history. The tension between science and vengeance sharpens as investigators close in on the truth.

Marie, Damas’ half-sister, is revealed as the true killer, driven by revenge and a craving for their father’s fortune. She injects the victims with poison and uses charcoal to draw black spots on their skin to mimic plague as the apparent cause of death. Her plan relies on misdirection and the social fear surrounding epidemics.

With the siblings' scheme exposed, Clémentine is admitted to psychiatry and Damas receives a five-year prison sentence. Marie’s revenge plot unravels, leaving the police to close the case on a note of tragic irony. The ending underscores how fear, inheritance, and symbol can conspire to mask a coldly executed crime.