Answer: Mme. Forestier reveals to her that the necklace was made of paste instead of real diamonds and wasn't worth much at all. Mathilde went into debt for no reason.
The story is "The Necklace" by Guy de Maupassant. It tells the story of a vain, superficial woman who borrows a very expensive diamond necklace from a friend for a party. She ends up losing the necklace, and spends the rest of her life paying the replacement she bought for her friend. Near the end, there is a plot twist: the friend tells her that the necklace was in fact made of paste and was completely worthless.
I know that you said 'opening line', but I found all of that information kind of hard to fit into a handful of sentences. As a result, what I've written might really differ from your perceptions of the story. I hope that it's not too different and that it works for you. Good luck.
