"It's Dark Outside" (1964-5) was a spin-off from an earlier series entitled "The Odd Man" (1962) which was an adaption of a series on radio about husband and wife amateur sleuths Steve & Judy Gardiner.
"The Odd Man" became a cult success in the UK. When wife Judy Gardiner was murdered in the premier season, Granada Television was deluged by angry letters and quasi-resurrected her with the same actress playing Judy's twin sister.
The second season saw the introduction of Inspector Charles Rose and it was this character which spawned "It's Dark Outside" and a further series "Mr. Rose" (1967).