Black Narcissus.
I first saw it when I was a schoolchild - I had no idea. I'm now an OAP - I still have no idea!!!!!!!
I did enjoy the series (though as a bit of a Gemma Arterton fan I'd probably have watched it anyway ), but ever since I heard it announced it felt a bit of a strange choice as a drama adaptation by the BBC.
Firstly, the Powell and Pressburger movie's a classic of British cinema, so they were always likely to be on a hiding to nothing anyway. But also the psychological "basis" behind the story seems rather out-of-date to modern eyes (the original book was written in 1939, and was a bit melodramatic even then... ).