M. Loisel is introduced as the "little clerk in the Department of Education" Mathilde's family marries her off to (1). Mathilde doesn't seem too happy about the whole thing.
We learn, from a telling comparison about how they eat their dinner that M. Loisel is quite content with his life, while Mathilde is not.
M. Loisel comes home with an invitation to a fabulous ball his boss is throwing. Mathilde surprises him by having a fit because she doesn't have a dress. He agrees to buy her a new dress. There goes that hunting rifle for which he had been saving.
M. Loisel finds his wife (who's bought the dress) upset again only a few days before the ball. This time, she's upset that she doesn't have any jewelry to wear with the dress. He tells her to borrow some from Mme. Forestier, and she takes his suggestion.
M. Loisel goes to the ball with Mathilde, and sleeps through most of it. He leaves with her at 4am, and takes a cab back home.
M. Loisel is just as alarmed as Mathilde to discover the necklace is missing. After spending a while searching her clothes, he goes out to tell the police, the newspapers, and the cab company. He spends the next week searching. Nothing.
M. Loisel decides that he and Mathilde have no choice but to buy Mme. Forestier a new necklace. They find a replacement at a jeweler's shop.
M. Loisel gives up his life's savings and goes into massive debt in order to buy the necklace.
M. Loisel spends the next ten years repaying the debts, working as a book-keeper and a copier in the time that's not spent working for the ministry. After ten years, he pays off the last debt.