A robbery of 10 billion is successfully carried out by a Corsican gang led by a man called Toussaint. The heist is also the start of a war with a rival gang.

The Marquis Rodolphe de Sombrueil accidentally runs over a working man with his carriage and helps his widow -- unjustly accused of robbery -- to find her missing daughter.

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...!