Where do you even begin with this one?
I'll give two opinions, first on the mystery element, and secondly on the episode overall.
As a story, it's actually a pretty good one, plenty of red herrings, some interesting characters, and a few surprises, especially at the end. John Barrie as always is delightful as Sergeant Cork, his protestations at having to travel were very funny.
Made back in 1964, racial attitudes were of course very different, attitudes which fortunately have changed, though sadly still exist.
It's hard not to get a little angered by some of the dialogue, but I say again made back in 1964, where attitudes were very different.
Richard Brinton is a good character, with surprisingly sensible ideas, he's fully aware of the asset stripping that occured, quite liberal in his thoughts. Michael Hawkins is pretty good here.
Almost impossible to rate, 6/10.