She borrows a diamon necklace from her friend MAdame Forestier to go to the Chancellors ball. But when she comes home she finds out that she lost the necklace. Her and her husband later on bought a new one which was out of their price ranged. This lead them being into an enormmus debt that made them very depressed. But one day when Madame Losiel sees Forestier she explains to her what happend the night of the ball and tells her that she is very weak from all the work she does. And in the end Madame Forestier tells her, that necklace was a fake. And it was only worth five hundred francs.
Mathilde is full of pride and envy and this in the end causes her downfall. She borrows a necklace from a friend of hers that is better off than she is, then she loses the necklace. Rather than admit that she's lost it, she and her husband purchase another one and it costs them everything they have (including their health). Later, she finds out from her friend (when she finally does confess to losing the necklace) that the necklace was not worth anything to begin with.