Madame Loisel is upset because she doesn't have fancy things. She wants to be rich, have fancy things and go to rich parties.
Her husband gets them an invitation to a rich party. She complains about how she doesn't have a dress to wear. They get her an expensive dress, even though they can barley afford it.
She then realizes she needs a necklace to go with the dress so she borrows one from her rich friend. It is diamond and Madame Loisel is very excited about it.
Madame Loisel and her husband have an amazing time at the party. As they are leaving the party, she realizes she lost the necklace. She searches but cannot find it.She doesn't want to tell her friend.
So, she buys one exactly like it for thirty-six thousand francs that they borrow from multiple people. Madame Loisel and her husband work for ten years to pay it back. They move into an attic and fire their maid.
She sees her friend that she borrowed the necklace to on the street one day and decides that she's going to tell her the truth. Her friend then tells her that the original necklace only costs five hundred Francs.