CONTAINS SOME SPOILERS, INCLUDING THE REALLY REALLY BIG ONE TOWARDS THE END.
The IMDb page suggests that this was originally made as a pilot episode for an American TV show (which would explain its 4:3 ratio) but the intended series never materialised and the pilot was released to cinemas instead, which is slightly odd (although the pilots of both Battlestar Galactica and Buck Rogers In The 25th Century did make it to UK cinemas). It would perhaps be unusual to have a weekly TV series based on the exploits of the titular villain as they really should lose every time, or at the very least break even, so maybe they felt it wasn't a good enough idea in the long term even though it just about manages to get away with it for one solitary outing. In film it's probably easier - Dracula, a dozen slasher icons - but the leads in TV series almost always have to be at least on the side of the angels, even if they're not actually angels.
You would think that the focus is on cynical agent Tony Lawrence (Robert Wagner) rather than legendary master criminal Madame Sin (Bette Davis), given that she doesn't show up for quite some time. Like Fu Manchu, she lives in isolation but surrounded by her acolytes and goons, working on her latest fiendish plan: in this instance to steal a Polaris submarine from the nearby base. To achieve this she forces Lawrence to abduct the Commander of the Royal Navy (Gordon Jackson) so he can be brainwashed by one of Madame Sin's numerous gizmos. Can Lawrence escape or alert the authorities - particularly when rendered deaf by Madame Sin's sonic gun?
There are some pleasures, principally the star-spotting game (Dudley Sutton as an assassin, Roy Kinnear as a holidaymaker, and Denholm Elliott as Madame Sin's fawning Number Two), but really it's a load of tosh - the villainess's scheme is to brainwash a top Navy Commander into re-routing a nuclear submarine, by persuading his old friend, a disillusioned spy, to kidnap him, and she's also faked the torture and murder of our hero's girlfriend (Catherine Schell), apparently for no good reason. Nor is it enough of an absurd and overblown fantasy to get by as a sub-Bond spoof: on the contrary, it's all a bit drab.
And worse, there's the genuinely surprising downbeat ending, in which our hero is pointlessly killed off and Madame Sin gets clean away with her sights on Buckingham Palace and the Russian Crown Jewels, which just leaves you wondering where it was ever supposed to go as a weekly TV show. Madame Sin isn't a complete disaster, but it's only average at best and if you don't see it, you won't be missing very much.
Sin! Sin! Sin!