Set in Brussels, where Suzanne Beulemans, the daughter of a rich brewer is promised to marry Séraphin Meulemeester, the son of a rival brewer. The young man and his father both seem particularly motivated by the dowry of the young fiancée. But Séraphin has a rival in Albert Delpierre, a young Frenchman who is learning brewery from Ferdinand Beulemans and who is discreetly enamoured with the young woman. Albert learns Séraphin's secret that he is having an affair with a worker and that they have had a child. He promises Séraphin that he will never reveal any of it to Suzanne, but she is told by Isabelle, her maid.

A woman's marriage is on the rocks ; to avenge herself, she decides to take a lover for one night .So she buys all his stock from a balloon man and flies them through the Parisian sky ; all her balloons carry a message :"I will be alone after midnight" (hence the title); a lot a suitors comme to the rendezvous : a fisherman, a soldier, a traveler , a gentleman cambrioleur (a nod to Arsène Lupin?) and others ,much to Michel, a young man in love with her's displeasure.