Agency O is commissioned by an insurance company to investigate a series of burglaries that the police attribute to a black jacket that moves on a powerful motorcycle. In his office, Inspector Torrence is examining an object he found, during an investigation, in a jewellery store and whose nature and function are difficult to specify. Then a young brunette girl enters who comes to entrust Torrence with an important document. When the young girl leaves the agency, the object has disappeared...

The O Agency finds itself in Cannes to help "Grandmother" find a seducer and thief of mink coats.