Crime Unlimited (1935)

Frustrated by a seemingly infallible gang of jewel thieves, Scotland Yard arranges to have an undercover agent become part of the gang. Once inside, the agent starts to become attached to a Russian woman who is part of the gang but seems to want to break free of the sordid life she leads. Trying to trap the leader of the gang -- whom no one has ever seen - the police plan goes awry and their agent is put into mortal danger, with his one chance being the Russian woman having told the truth about wanting to break free.