Michel Piccoli plays a police inspector whose best friend is murdered on the orders of gang boss Charles Vanel. The inspector knows full well that Vanel is too crafty and well-connected to ever stand trial for his crime, so he carefully lays a subtle trap for his adversary. Unfortunately, both Piccoli and Vanel are thwarted by a pair of scheming females.

During the First World War, the Empyrée Montmartre, a Paris music-hall, is dedicated to patriotic revues whose star is the charming Mitsou. The young artist is not without talent but she is mainly well-connected. She is indeed the cherished mistress of Pierre Duroy-Lelong, a rich industrialist. One night, thanks to Petite-Chose, an ebullient singer-dancer and her co-star, she gets to know a handsome army, Lieutenant Bleu. Mitsou falls madly in love with him and Lieutenant Bleu is physically attracted to her. The trouble is that Bleu comes from a distinguished family and cannot put up with her lack of culture and artistic bad taste...