I love these obscure support feature movies on Talking pictures. Usually set in the fifties, they offer the chance to wallow in the nostalgia of real period detail such as old British cars, wired telephones and everybody smoking like chimneys!
In there also is usually a neat little story and this is no exception. First surprise is everyone's-favourite-fifties-nazi Anton Diffring playing the titular debonair French detective, who has come over to help the Brit plods track down the notorious jewel thief, and murderer, Mr March. A young woman is strangled and her diamonds nicked. The police have suspects but little to go on. Fortunately for the audience we are allowed insight via the activities of some of Mr March's accomplices. Due to the obviously limited budget, this seems to include most of the cast! Inspector Duval applies his Gallic charm to both male and female suspects and appears to be working towards a solution. However, as players are discounted one by one, the culprit unfortunately becomes rather obvious.
As someone else has remarked, the title gives the impression that the movie could have been the first of a series. In fact with a little bit of trimming it could have been a t v cop show episode. Nice to see Anton Diffring in something other than an SS uniform. Pity he didn't do more such roles.