Under the regime of Louis XVIII. Years after Waterloo, Napoleon's loyal officers live in retirement on half-pay. Useless and idle, they have lost their prestige and keep meeting in the hope of the Emperor’s return. When Napoleon dies, they decide to plot the rise to power of Napoleon II.
Monsieur Loyal, bailiff by profession, is dispatched to a grub by an actress. Reason: she accuses Micheline, a young singer who works there, of stealing her songs. But our usher is nonetheless a man, and Micheline exerts her charms on him so well that he gives him shelter for the night, and finds him a new job at the Mirador, a cabaret that has just opened.
Lieutenant Varnière, responsible for discovering the cause of the death of two officers in Algeria, unmasks the assassin in the person of a Russian who lives with three women. He falls in love with one of them and runs away with her while the Russian is killed by a devoted servant.
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.
On his wedding night with Hilda Bergmann, the daughter of a swedish chemist, the young and rich american Willy Keanton is stabbed by an unidentified masked man. He kidnaps the young woman, who is found later on by the count Robert d'Arbel. The detective Jacques Miral, nicknamed « the pointed tower's ferret » is put in charge of the case. Soon, he discovers that an international bandit, Méphisto, seeks to seize the formula of a process against asphyxiating gas, invented by professor Bergmann. The struggle is tough between Miral and the uncatchable Méphisto. Monique Aubray, Keanton's secretary and engaged to the detective, the Nostradamus stallholders and the « mastiff of Bordeaux », and the famous novelist Fortuné Bidon are also involved in the case...
A bank employee who stole money at work falls in the hands of a dangerous escaped convict whose girlfriend is going to help him out of this mess.
In the tenement slums of Paris between the world wars, impoverished street singer Albert yearns for beautiful Romanian immigrant Pola. Pola's boyfriend, local hoodlum Fred, grows jealous of Albert's constant attention to his woman and frames the hapless musician for one of his own petty crimes. But while Albert is in prison for Fred's misdeed, Pola ends up falling for Albert's faithful best friend, Louis.
The whimsical Riquet's has kidnapped and married a well-born young girl, Ralda. They travel around Spain performing their act in a circus but Ralda's beauty arouses the director's lust. Furious at being rejected, he opens a lion's cage during Ralda's act, leaving her seriously injured. With Ralda's family on their trail, the young people set off to find a better life elsewhere.