When I was growing up, Sapphire and Steel was a revelation to me. I had never seen anything like it before.
In many episodes, nothing much really happens, characters are scant and background information just isn't touched on at all sometimes.
But I was fascinated and scared, attracted and appalled. Two assignments that stood out for me was the Railway Station and the Man with No Face. Both very creepy - and fear was more what you didn't see, not what you did.
As I grew older, there seemed to be little else like it on TV, until The X-Files. Now of course, there is endless mysteries with unknown forces at work (supernatural, alien, whatever the cause), but none of them seem quite the same. Maybe it was the very Britishness of it all that added to the appeal.
The lead characters too were appealing, both looked great and both played well.
It was one of those series that for me, ended too soon.
(My final and only caveat was that in recent years, I watched a few episodes, and found them sadly disappointing; if I had not, my memories would have been untarnished).