I do not know if I ever mentioned it but way back, yes I know it seems too soon to talk like this but years are years, during the first season of Peter Capaldi's era as The Doctor, there was that season long mystery with regard to the identity of Missy. Just who was she?Actually I thought I had it sussed. Old clever boots me. I thought it would turn out that Missy was Romanna , an evil version of The Doctor's best friend. A damaged and war-traumatised version of Romanna twisted by the terrible events of that horrific conflict. Hurt but redeemable under the influence of of her own best friend to remind her off better times and happier days. Watching Shada and Lalla Ward's star turn brought it back to mind. Perhaps you too can see why when you check out these two pictures.
I was wrong off course. I frequently am. My theories are nearly always well off. Missy was The Master, in a worthy piece of foreshadowing that like so many Conan Doyle resolutions seems so obvious now. Therein lies the genius I suppose...