The movie title refers to a gang of six criminals that Scotland Yard is unable to pin a charge against even though they are positive of the identities.
Throughout the film the gang members are, one by one, set up and framed with a charge that ensures a decent jail term who is pulling the strings for this to occur? It seems the only link is the 'femme' Christina, but how and why this would be an answer is not clear until the final few minutes when the whole mystery is unraveled.
This is a 'B' movie time waster and will not appear in any desirable list. The actors try hard and for the most part do very well with the available material. It is certainly worth an hour of your time, particularly if you enjoy the British B movie Crime / Drama genre.
7 /10 from me