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Marie Lemaire is poisoned with fentanyl at a dinner for an association of tradespeople. Her business was the largest glassmaking firm in town, inherited from her recently deceased husband, François.

Marie Lemaire is poisoned during a dinner that brings together an association of merchants. She is the heiress of her recently deceased husband, François, who was the owner of the largest glass factory in the city. All the guests are potential suspects and the investigators must sort out the real from the fake, detect motives and verify alibis. Olivia Rousseau, one of the guests, ex-mistress of François Lemaire who left her to marry Marie, is dead at her home. In a letter, she declared that she had poisoned Marie and killed herself in the same way.

Marie Lemaire, widow of François Lemaire, is poisoned in a business dinner and dead. Prosecutor Elisabeth Richard and Gendarmerie Captain Charles Jouanic are charged with the investigation. One of the guests Clémentine Roux was witnessed to arguing with Marie, but she claims they quarreled because Marie decided to withdraw from their plan of running a guesthouse together. Another one of the guests Olivia Rousseau, ex-wife of François, who left her to marry Marie, is found dead and leaves a suicide note declaring that she poisoned Marie because she blamed Marie for murdering François. Soon François's nephew Hugo Fontaine is assaulted in an attempted murder and sent to hospital. The investigators learn from Fontaine's fiancee Julie Caron that Fontaine contacted allegedly Marie's psychiatric clinic and found out the psychotic one is Roux instead of Marie. Roux killed Marie for she has paranoid schizophrenia of being in love with Marie and of being abandoned. Next, she has Rousseau to be the scapegoat. Moreover, after she realizes her secret is out, she has to silence Fontaine and Caron. When Roux is trying to kill Caron, the investigators stop her in time and save Caron.