Terror of the Zygons is the finale of Season 12 of psuedoscience series Doctor I Say Now Who's That Again, except that it was delayed so it became the opener of Season 13 instead. It features The Scarf, Only Works on Sailors, and Sailors Only Wish, helping UNIT chase the Loch Ness Monster from one end of Britain to the other.
Also, the Doctor makes Zygons be bygones. Yeah, I know, it's not that funny, but that's how TV Guide described it in 1979, so every American who was a kid at the time is legally obligated to quote it every time this story is mentioned. The Zygons would later go on to star in porn for BBV.
"Nessie in the Thames" is also the only example Moffat could think of from Classic Who of an alien invasion that couldn't have been covered up and therefore must have been erased by Amy's crack.
There's a bunch of local Scottish politics about titled landowners vs. commoners that the Brits still hadn't resolved by the late 1970s, but the Brig's got national secrets and a fuckload of Englishmen with guns on his side, so he tells them all to shut up, and somehow that's cool even though it'll be evil when Margaret Thatcher does it in real life a year later.
But not before Harry gets shot, so he gets to play doctor from the other side of the table. As payback for all the times he called Sarah Jane "old thing" he doesn't get a hot nurse.
Then an orange hand with suction cups on grabs Sarah Jane. Cliffhanger!
The Doctor rushes in to save Sarah Jane and she's gone. He finds her in a compression chamber, but discovers that Harry is really gone. The Zygons left the girl, it's the man they want.
Then the Zygons lock the compression chamber and suck out all the air. The Doctor hypnotises Sarah Jane to believe she doesn't need to breathe. This works. Then he saves himself with Buddhism. Later, Benton saves them, so the three of them are the only ones left awake when the Zygons sleep-gas the town.
The Zygons, meanwhile, kindly give Harry an infodump. They crashed on Earth, and their rescue ship never came, and then their home planet exploded, so they're taking this one. Also, they're shapeshifters—as long as they have someone's body is hooked up to their system, they can take that person's place. Also, the Loch Ness Monster is really the Skarasen, and all Zygons survive by sucking Skarasen tits. Wait, are you sure this isn't the Zygon porn one?
The Doctor, Benton, and Sarah Jane explore and end up with a button that reproduces the mating call of the Skarasen. OK, this isn't the Zygon porn one, it's the Skarasen porn one.
Everyone wakes up, and fake Zygon Harry takes the button from Sarah Jane, but decides to act like an asshole, push Sarah Jane, and run off, so all of UNIT chases him into a barn, where he accidentally commits suicide, which frees the real Harry from the body print machine, and gives the Doctor back his button. He uses it to lure the Skarasen away, but the button fuses itself to his hand, so now nothing will stop the Skarasen from raping him to death. Cliffhanger!
Harry hits a bunch of random buttons, and one of them was the "unstick the button from the Doctor's hand" control, while the others had no function at all, so the Doctor is able to avoid being raped to death, but the Zygons don't bother to check, so they just assume he's dead and they've won.
Various people turn out to be Zygons, including Harry's nurse and the Duke from the beginning. Sarah Jane, Girl Explorer, does some girl exploring and finds that the Duke's castle is secretly connected to the Zygon ship, and frees Harry, but the Doctor manages to get himself caught, and the Zygons decide they don't need any prisoners besides him anyway. Benton saves the Doctor, finally doing something other than being the straight man for the Brig and Yates to make gay jokes at each other's expense, and he's pretty good at it.
The Brig decides the only solution to this problem is to drop depth charges all over Loch Ness to force the Zygon spaceship to rise. Which is does, but then it flies off, being a spaceship and all. Cliffhanger!
Sarah Jane continues searching the castle and discovers that the Duke was head of the Scottish Energy Commission. Harry says that's useless information, you're a girl, girls are useless. So they all go off with UNIT.
The Zygons land in a quarry that the BBC has cunningly disguised as an actual no-shit English quarry for once. This time they give the Doctor an info-dump. A Zygon refugee ship is coming, so they have to get Earth tech up to spaceworthy level before it arrives in a few centuries, and humans (and The Silence) are too stupid to do it in time, so they have to take over the planet now. Then they leave the Doctor alone to run around, so he signals UNIT, frees all the body print people so the Zygons can't shapechange, and activates the ship's self destruct sequence so all the Zygons die.
It turns out that, because the Duke is head of the Scottish Energy Commission, he has to go to London for an energy conference. Suck it, Harry. So UNIT escorts him there, meaning everyone's there just in time to see Nessie emerge from the Thames. See, the Zygon leader survived, and he has a Skarasen control thingy, and in Doctor Who Scotland is just upriver from Ealing, so he's going to have the Skarasen eat the PM and some energy commissioners to prove his power to the world and make everyone surrender.
The Skarasen is immune to bullets, but Zygons aren't, so the Brig just shoots the Zygon.
The Doctor then tosses the control thingy to the Skarasen, which eats it and swims back to Loch Ness to be a tourist attraction. Everyone goes back to Scotland, Harry decides to leave the TARDIS, and the Doctor makes an ethnic joke about the Brig being Scottish and therefore cheap. The end.
Every Classic Who fan believes this story is pretty good, but overrated by all the other fans.
The Zygons' plans are all dead stupid. The Skarasen is giant-rat-level bad. The entire plot is borrowed from Doctor Who and the Silurians, hitting the same beats in the same order. The Zygons say things like "I underestimated his intelligence, but he underestimated the power of organic crystallography." And every time they seem scary, the Doctor shows up with a funny line to remind us that there are only six of them. The Scots are recognizable as insulting stereotype Scots even to a 9-year-old who's never been closer to Scotland than Canada. And the ending fizzles badly, being entirely dependent on the Skarasen being scary, which it isn't.
On the other hand, the Zygons look good for 70s rubber suit monsters, and their organic tech made of pepperoni pizza is suitably creepy. They're also a cool concept for a monster, well fleshed out. (David Tennant isn't always wrong.) Harry actually gets something useful to do for the first time (in his last story), and Ian Marter makes the most of it, while Sarah Jane reminds us of how useful she is in Earth-based stories. Finally, UNIT's swansong is exactly what it should have been—the Brig finally getting to shoot an alien who's not immune to bullets.
Also, Camfield directs the fuck out of this thing, giving us the action of the Letts UNIT era and the horror of the Hinchcliffe-Holmes era without letting either one dilute the other, and using the slower moments to build suspense instead of to bore us to sleep.
So, yeah, it's pretty good, but overrated by all the other fans.