Paris is a feast, and the lights of the Belle Époque illuminate the nightlife of Montmartre and Pigalle, neighbourhoods of loose morals and the free circulation of absinthe and opium. But it is in an aristocratic boulevard, far from the backstreets and dens, that the body of Monsieur Bonancieux appears with its throat cut. There are no clues, or obvious motive, despite the fact that the gentleman had a secret passion: he had a hidden alchemical laboratory in his home, and had dedicated his life to the search for the philosopher's stone. Within a framework of false appearances, obscure formulas and unconfessed desires for the possessions of others', only two things are certain: the first, that a man can reveal many surprises after his death, and, secondly, that no one ever knows who his neighbours really are.