Sophie Mallet is the lively manager of a detective agency, assisted by Emile Gachit, her conscientious but not very effective right-hand man. One day a fellow named David entrusts her a pack of cigarettes that she is supposed to hand over to a certain John Smith. But David soon disappears while John Smith is murdered. Moreover, a lot of dubious characters prove "very interested" in the pack of cigarettes... Which will keep Sophie and Emile for some time...!

A la suite d'une nuit d'amour avec un Anglais, une jeune detective est poursuivie par de sinistres individus a la recherche d'un microfilm.