Metro Ticket

Shot by a gun, a client comes to die in the offices of the O Agency. He is a comfortable bourgeois from Saint Etienne who was due to marry his son the next day. The only clues: a metro ticket, and in his pocket a large revolver that was used a short time ago. But why did he regularly receive boxes of watercolors in the mail when he wasn't painting? When his last words, more or less intelligible, are finally understood, the evidence stands out.