It's a party and I'll cry if I want to…
The action proper begins when M. Loisel (Mathilde's husband) comes home with the invitation to the fabulous ball and Mathilde reacts by having a fit. Now we have a specific problem: Mathilde's now has the best opportunity she's ever had to have a taste of the high life, but she has nothing to wear. That problem sets the rest of the plot in motion.