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.

Simon's spontaneous decision to help a stranger embroils him in a murder plot.