Set in Paris during the Roaring Twenties, La Garçonne follows Louise Kerlac (Laura Smet, Yves Saint Laurent) who witnesses a murder. A career forbidden to women at the time, Louise is convinced that joining the police will be her best hiding place and provides the means to investigate the murder.
She therefore assumes the identity of her twin brother Antoine (Tom Hygreck, Valerian) and enters the police force as a man.
In order to infiltrate the Parisian bohemian nightlife and to protect her identity, she creates another character, a flamboyant and glamorous female alter ego of herself, Gisele. Louise is now investigating as a man by day and as a woman by night. These two investigations will shatter the core of her existence...