Just watched Terror of the Zygons. Holy cow! It's one of the only serials that I've managed to remain near-wholly unspoiled on (the only other may be The Talons of Weng-Chiang).
It's clearly the finale for Season 12, and really should have been kept there. Nessie was a letdown, but it was good that they didn't use the model too much - it helped disguise the...inadequacy. The whole first episode was perfect, especially the Doctor's annoyance at the Brigadier calling him back. It really helped establish why the Fourth Doctor worked much less with UNIT than the Third Doctor. Sarah, Harry, Benton, and the Brigadier all had great moments, and it's a great shame that this was Harry's last regular appearance. Really wish they'd kept him on.
The Zygons themselves were a bit disappointing, as their plan was blatantly obvious in the middle of the second episode. But they're definitely distinctive, and I'm surprised that they were never brought back in another Classic serial. One wonders if the costumes degraded in storage; they were quite well-done.
And, of course, it feels like Douglas Camfield's swan song. A much better story than The Seeds of Doom. Great script, great acting, and fantastic direction.
I knew that a scene had been added as a Director's Cut, and chose to watch the first episode with it added. Fantastic restoration work by Babelcolour. Like The Mind of Evil's first episode, if I hadn't known beforehand that it was recolorized, I never would have guessed.
All in all, it's clearly one of the best Fourth Doctor stories, and I'm kinda glad they kept it for the very end of the DVD range. Obviously, this and The Tenth Planet were supposed to be the grand finale, and it would have brought the range to a much more satisfying end than the VHS range had. Of course, the recovery of missing episodes torpedoes that plan, but it's still a good decision in hindsight.