Mathilde is married to a loving, poor, contented clerk. She, on the other hand, is not content. She feels as if she belongs to a higher social class. She spends her time daydreaming about being rich and having more lavish clothes and jewelry. She wants to be envied. She wants other people to look at her with awe. Her husband is content with their life together. She feels empty and a sense of longing. But her sadness has to do with a selfish love for material wealth.
When she and her husband are invited to a fancy ball, she is upset because she has nothing expensive to wear. Her husband reluctantly gives her the money he'd set aside for himself so she can buy a nice dress. She complains that she needs some jewelry to go with it. He suggests that she could borrow something from her friend, Mme. Forestier. She borrows a diamond necklace. They go to the ball and Mathlide is the hit of the party. Wearing the dress and diamonds changes her attitude. She gets her wish.
They get home and she realizes she has lost the necklace. Her husband spends all night looking for it but finds nothing. They borrow money to have the necklace replaced and spend ten years paying off these debts. Mathilde learns the true nature of begin poor and working hard. The ten years are hard on her. She runs into Mme. Forestier at this point and discovers that the necklace was paste and therefore much less expensive than the forty thousand francs they paid for the real diamond necklace.