Invited to a ball at the Ministry of Public Instruction where her husband works, Mathilde is in despair about how to look the part. She manages to persuade her husband to give her 400 francs for a new dress. But she also would like some jewellery to complete the outfit. Although their budget cannot stretch to this, Mathilde is able to borrow a diamond necklace from an old schoolfriend who is wealthy. But while she dazzles everyone at the ball, disaster is waiting in the wings...