Mme. L borrows a necklace to fit in with the rich people she envies, but the loss of the necklace results in her being poorer than she was.
After Mme. L loses the necklace & must repay the debt, she endures 10 years of hardship only to find out the necklace is a fake.
Mme. Loisel's intense desire for a different life is then equally matched by her intense suffering.