Miserable Mathilde
At the beginning of the story, essentially nothing happens. The narrator's interested in telling us about Mathilde (even though we don't yet know her name). We learn about her back-story, her character, and her unhappiness with her mediocre life. This represents a classic initial situation.
Conflict
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.
Complication
Diamonds are this girl's best friend
Mathilde solves the first problem when her husband gives her money for a dress. But then she runs into a second problem: she's needs to have some jewels. Luckily, her friend Mme. Forestier is able to provide her with a fabulous diamond necklace. But now Mathilde's been entrusted with something expensive that belongs to someone else and we have the potential for disaster. It's true that the complication is often when things "get worse," and that doesn't really happen here (for that, we have to wait for the climax). In fact, after borrowing the necklace, Mathilde has the time of her life. But it's when she borrows the necklace that the possibility opens up for something really bad to happen…and it does.
Climax
The necklace is missing!
Mathilde's discovery is the most exciting and dramatic moment in the story (until that crazy twist in the last line). It's also the turning point in the plot. Before, the story was a build-up to Mathilde's one glorious night with the rich and famous. Now it transitions into a desperate search. We have a feeling things are not going to end well.
Suspense
Diamonds, when lost, are a girl's worst nightmare
After the loss of the necklace, we're kept in constant