One large metaphor from the story is the invitation that Monsieur Loisel brings home to his wife. It is literally a ticket into a party where all the "really big people" will be, but it is metaphorically a ticket into 10 years of hard labor and degradation.
Madame Loisel felt trapped in the wrong life; she dreamed of having all the finer things she deserved because of her fine looks, grace, and charm. But, alas, she lived a modest life as the wife of a clerk in an apartment with one maid. Upon learning of the invitation to a party hosted by the Minister of Education and his wife, Madame Loisel insists upon procuring proper attire to wear amongst such fine people; this includes wearing real jewelry. After borrowing one of Mme. Forestier's diamond necklaces, she is excited to attend the party and mingle amongst the people whose class she so desperately wishes to be a part of. The night of the party, though, ends disastrously. Mme. Loisel has misplaced the necklace; she makes an exhaustive search for it, but is unable to find it. Her husband and she must replace the necklace, which requires her using her entire inheritance, borrowing from friends and crooks, and going into horrible debt. She and her husband dismissed their maid and moved into a smaller apartment.
They had to adjust to a life of "abject poverty." Her days filled with menial duties of cleaning, cooking, and going to the market. After meeting Mme. Forestier 10 years into this new life of always going without, the truth is unveiled: the necklace had been a fake.
The invitation to the party, then, welcomed Mme. Loisel (and her husband) into a hell of an existence.