The Adventures of P.C. 49: Investigating the Case of the Guardian Angel (1949)

Hugh Latimer is the eponymous officer who manages to convince his inspector to take him off his uniformed beat and go undercover to track down a gang of violent thieves. It's based on an old BBC radio series, and his criminal escapades are gentle and engaging - though maybe not the most difficult to grasp. This one has a few red herrings as we try to discover who the "boss" of the gang led by the dastardly John Penrose is, and although Latimer is pretty wooden there are some other good characters "Skinny". "Ma" and "Carrots" amongst them, and the whole hours flows quite well with just enough peril to keep it interesting.