In Uncle Leon and Aunt Lucienne's pharmacy, every night, drugs disappear. While her aunt and uncle move heaven and earth to find the culprits, Jeanne is more worried about the health of the thieves, and decides to conduct her own investigation! Thus begins a race against time in which the little girl finds herself in all sorts of unlikely places, from the catacombs to the artists' neighborhood, meeting Kiki de Montparnasse and befriending a group of street children...