At the beginning of the story, we meet Mathilde Loisel, a middle-class girl who desperately wishes she were wealthy. She's got looks and charm, but had the bad luck to be born into a family of clerks, who marry her to another clerk (M. Loisel) in the Department of Education. Mathilde is so convinced she's meant to be rich that she detests her real life and spends all day dreaming and despairing about the fabulous life she's not having. She envisions footmen, feasts, fancy furniture, and strings of rich young men to seduce.
One day M. Loisel comes home with an invitation to a fancy ball thrown by his boss, the Minister of Education. M. Loisel has gone to a lot of trouble to get the invitation, but Mathilde's first reaction is to throw a fit. She doesn't have anything nice to wear, and can't possibly go! How dare her husband be so insensitive? M. Loisel doesn't know what to do, and offers to buy his wife a dress, so long as it's not too expensive. Mathilde asks for 400 francs, and he agrees. It's not too long before Mathilde throws another fit, though, this time because she has no jewels. So M. Loisel suggests she go see her friend Mme. Forestier, a rich woman who can probably lend her something. Mathilde goes to see Mme. Forestier, and she is in luck. Mathilde is able to borrow a gorgeous diamond necklace. With the necklace, she's sure to be a stunner.
The night of the ball arrives, and Mathilde has the time of her life. Everyone loves her (i.e., lusts after her) and she is absolutely thrilled. She and her husband (who falls asleep off in a corner) don't leave until 4am. Mathilde suddenly dashes outside to avoid being seen in her shabby coat. She and her husband catch a cab and head home. But once back at home, Mathilde makes a horrifying discovery: the diamond necklace is gone.
M. Loisel spends all of the next day, and even the next week, searching the city for the necklace, but finds nothing. It's gone. So he and Mathilde decide they have no choice but to buy Mme. Forestier a new necklace. They visit one jewelry store after another until at last they find a necklace that looks just the same as the one they lost. Unfortunately, it's 36 thousand francs, which is exactly twice the amount of all the money M. Loisel has to his name. So M. Loisel goes massively into debt and buys the necklace, and Mathilde returns it to Mme. Forestier, who doesn't notice the substitution. Buying the necklace catapults the Loisels into poverty for the next ten years. That's right, ten years. They lose their house, their maid, their comfortable lifestyle, and on top of it all Mathilde loses her good looks.
After ten years, all the debts are finally paid, and Mathilde is out for a jaunt on the Champs Elysées. There she comes across Mme. Forestier, rich and beautiful as ever. Now that all the debts are paid off, Mathilde decides she wants to finally tell Mme. Forestier the sad story of the necklace and her ten years of poverty, and she does. At that point, Mme. Forestier, aghast, reveals to Mathilde that the necklace she lost was just a fake. It was worth only five hundred francs.