That was good. Janet Fielding is truly great in this even if Tegan is a bit of a victim throughout. As much as I like the idea of Nyssa and think she's pretty, Sutton really still is pretty bad and her acting reminds me a bit of Zoey from the Troughton years...variable and at times really poor, at times charming. Her inability to get the Doctor to be interested in her new dress is so ill timed and misplayed by Sutton that all the subtly and humor goes out of the situation. It could have been so well done and it's really a take on a similar pair of scenes in a Victoria story where I believe the Doc and Jamie do notice and then later when Sarah Jane tries on the same outfit and the Fourth Doctor notices barely and calls her Vicky or Victoria (I think, wrongly he calls her Vicky and then doesn't even really look at the dress). Davison does his best in this scene and it works from the Doc's perspective but not from Nyssa's really and it's really due to Sutton's inability to act to be honest and IMO.
At the time I first saw this I thought Lon was a bit of a tranvestite but recently viewing it I could see he's a slacker lay about who happens to wear make up and lipstick and walk around barefoot in his house, fingering a snake statue. The build up was rather well done and all the characters seem well defined. The sets are pretty good too. There are long sequences in the TARDIS and that's always welcome if they're as good as these, apart from the lone Nyssa dress sequence.
Not sure why the Doctor thought leaving Tegan alone with Nyssa would be a good thing nor why letting her out of the TARDIS was a good thing when he knew the Mara was inside her. But then there has to be a story. H'e's pretty ineffective explaining things but then again, so were all the other Docs at times. It's just...he's so ineffective so much of the time but for once, he looks good doing it and also...he seems to have some presense finally and some authority.
The cliffhanger is most effective and creepy with the Mara skull in the crystal ball and the gypsy women at the market...and the market reminds me of the much later TURN LEFT market. Here, though, the gypsy woman is a victim and not a villain but she doesn't die, at least not here or at all if I recall correctly.
So all in all, a good episode, well done, tense, and interesting and different...oh, and a lot less techno babble.