Mathilde Loisel thinks she is too poor to go to an upper class party. She doesn't have a dress to wear. Her husband uses the money he's saved for a rifle and buys her a dress.
She didn't have any jewelry, so she went and borrowed a necklace from her rich friend.
She went to the party and lost the necklace.
She felt so bad. Her husband looked everywhere for it, but had no luck.
She felt like they needed to replace her necklace. Her husband borrowed 40,000 franks to buy a new necklace. She returned the necklace and never told her friend it was lost.
Ten years later, she saw her friend she borrowed the necklace from. Mathilde Loisel had aged a lot over the years paying off the loan. When she told her friend the story, her friend said the diamond necklace Mathilde Loisel borrowed was a fake.