To receive his fiancée and his father at his home, an Oxford student counts on the presence of his aunt from Brazil to serve as his chaperone. The latter having delayed his trip, the young man passes his servant off as his aunt. This results in multiple misunderstandings. Finally, everything works out when the aunt comes.
Jacques de Nisson, a rich man, has a new mistress, Lucie Cavelier. Which does not prevent him from courting Florence, the wife of his lawyer friend Forestier. One day, Jacques is found murdered and chief-inspector Candély is sent on the spot to investigate. Jean, the victim's butler, Lucie as well as Florence are suspected in turns but the real murderer is finally identified thanks to his fingerprints.
Démonio the fakir has impressed a rich but weak-willed young man who hires him as an energy teacher. Demonio drags out the businesses of the shy lover, but his partner arranges everything and the finally satisfied lover endows the two accomplices who renounce fakirism.
Escaping from a policeman who has caught h.im with a stolen dog, Toto (Albert Préjean), a petty crook, hides in the apartment of a typist, Ginette (Renée Saint-Cyr). The two quickly fall in love, and after a brief incarceration, Toto returns to her and schemes to have her win a beauty contest.
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.