It is one thing to open a beauty salon (which Edwige and her young lover Gaston have just done) but it is another to keep it on its feet.To best promote their speciality, fountain-of-youth treatments, Edwige decides to apply to the letter the old slogan "It pays to advertise" by posing as... Gaston's mother, a sixty-year-old woman, miraculously grown younger.
Professor Bock is to inaugurate the museum, the construction of which was donated by a generous patron who remains anonymous. He learns that it is his brother's wife, a former dancer, and whom he hates. But her daughter falls in love with Jim, the son of the ex-dancer, and everything will work out.
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.
The film is the first of a series of films containing the character of "Bouboule", created by the singer Milton and comprising La Bande à Bouboule (1931), directed by Léon Mathot, Le Roi du cirage (1931), directed by Pierre Colombier, Bouboule Ier, roi des Nègres (1933), directed by Léon Mathot, and Prince Bouboule, directed by Jacques Houssin (1939).
Mathilde Strangerson, the daughter of an eminent scientist, narrowly escapes being murdered in her own bedroom by an unknown assailant. As the room was locked from the inside, no one can understand how the attacker managed to enter or leave the room. Reporter and amateur sleuth Joseph Rouletabille arrives on the scene to protect Mathilde and resolve the mystery of the yellow room.