You can't rule the world in hiding. You've got to come out on the balcony sometimes and wave a tentacle.
After watching this story it is easy to see why the Zygons were one of the most commonly requested Classic Who villains. They look fantastic and they have a serious invasion of the body snatchers-type vibe going on as they kidnap humans and change their appearance to replace them. Pitted against the Doctor, Sarah and Harry - the ultimate TARDIS dream team - they Zygons prove to be a formidable threat. What makes this story so good is the nature by which the Zygons attack. Their monster - the mythical Loch Ness Monster - destroys oil rigs in the North Sea and later rampages down the River Thames but the body snatcher element is far more interesting. At points in the story you don't know who is good or bad, even the Doctor's own companions. Tom Baker is on fine form here as he works with the Nicholas Courtney's Brigadier for the last time to defeat the Zygons and he's clearly having lots of fun along the way. What makes this Doctor so engaging is his ability to be ruthless, intelligent and cunning but still find the time to make a quip or two along the way.