Madame Louisel gets invited to a dance with her husband while they are sitting down for dinner. She is very excited because she has never been to one before.
Madame Loisel doesn't have a dress to wear so she complains to her husband and he gives her 400 francs that he saved up to buy a gun to her so she could buy a nice dress. Then she complains that she doesn't have any jewelry to wear with her dress so her husband advises her to ask her friend to borrow one and she picks out a beautiful diamond necklace.
She had a lot of fun at the dance and she was very desired. She was the most beautiful and charming lady there. She was very happy because she had never been treated this way and she loved it.
Somewhere along the way home, she unknowingly dropped the necklace and lost it. She panicked and she had a choice between telling her friend the truth or borrowing the money and sacrificing their whole lives to buy a new necklace.
She then decides to lie to her friend and says the necklace needs to be repaired. Her and her husband borrowed a bunch of money to buy the new diamond necklace.
They bought the almost identical necklace and gave it to her friend . They had to move homes due to money and they worked extra jobs for the next 10 years to pay off all of the money that they borrowed. Then they later find out when Madame Louisel confessed, after all of their hard work, from her friend that the necklace was only a paste worth only 500 francs and that they did all of that for nothing.