Here we have Mathilde sitting with her good friend, Madame Forestier, at her friends house. Mathilde is disappointed and embarrassed that her own house is not as luxurious, because Mathilde believes she deserves better.
Mathilde's husband tries to be nice and gets her an invitation to a fancy party, but since Mathilde complains about having nothing to wear, her husband allows her to use his savings to buy a nice new dress and he tells Mathilde to borrow jewlery from Madmae Forestier.
When Mathilde visits her friend, she has plenty of choices to choose from. However, Mathilde being herself obviously picks the most expensive-looking necklace her friend owns to wear to the party.
The day of the party has arrived and Mathilde has such a nice time that she doesn't notice that the necklace goes missing.
For the next 10 years Mathilde and her husband spend their lives paying back banks and people they loaned money from so that Mathilde could pay for a necklace remake that costed thousands of dollars.
A few more years go by and Mathilde sees her old friend at a park with her son. Mathilde decides she might as well tell her friend all about the necklace and finds out the original necklace isn't even worth one thousand dollars.