The story opens with a long description of how wretched Mathilde is with her middle-class life. All she wants is to be rich, glamorous, and desired by men. It seems as if this will never happen.
Mathilde finally gets a chance to taste the life she's always dreamed of: she gets the invitation to the Minister's ball. And now she starts to move a bit beyond her normal circumstances: she gets her husband to buy her a genuinely nice dress, and borrows a gorgeous necklace from a rich friend (which will set her up for disaster). Then she goes to the ball, and is the most popular woman there.
Mathilde's trip to cloud nine comes to an abrupt end when she discovers she's lost the necklace. After a week of searching, it still doesn't show up, so and her husband decide to find a replacement. But now she and her husband are in a real crisis: how can they afford to replace it? Answer: they can't.
From one perspective, Mathilde's life is now worse than it's ever been – it's a nightmare. She's gone from an unremarkable but comfortable life to a grueling and difficult one. And this drags on for ten years. From another perspective, Mathilde's poverty forces her to start working hard, and she does. The debts get paid off. And Mathilde seems to have grown from the experience. It doesn't look as if this will actually have to be a tragedy.
Oh wait…it is a tragedy. So much for a happy ending. No one dies, but that doesn't make the tragic twist any less devastating.