The Doctor, Sarah Jane, and Harry arrive in Scotland near the North Sea, where Brigadier Lethbridge-Stewart and UNIT are investigating the destruction of oil rigs. A survivor claims they were destroyed by a sea creature, and indeed the Doctor finds giant tooth-marks in the wreckage.
Harry is captured by the Zygons, an alien race of shapeshifters from a submerged spaceship, which is powered by dynacron thrust and organic crystallography. Their leader Broton (John Woodnutt) tells Harry they crash-landed centuries ago and were waiting for rescue until they learned their home planet had been destroyed by an exploding star. So, they intend to conquer Earth and terraform it. They have captured several humans to use as body-prints to take over key leadership positions, including the head of the Scotland Energy Commission, the Duke of Forgill (also played by John Woodnutt). They also have a giant sea-creature called the Skarasen, which they have enhanced with cyborg tech. They keep it in Loch Ness and use it to attack the oil rigs.
Investigating this connection, Sarah Jane finds a secret passageway at the Duke’s mansion, which she follows to the Zygon’s submerged spaceship. Broton picks up the pace on his plan. appearing as the Duke, he leaves for London, while the rest of the Zygons fly the ship to a nearby quarry (can’t have Doctor Who without a quarry) to start their reactors to poison the Earth’s atmosphere. The Doctor sneaks aboard the ship, frees the human prisoners, and sets the ship to self-destruct.
One of the rescued humans is the real Duke of Forgill, who warns that he was scheduled to attend the First International Energy Commission in London, which a number of important dignitaries will be attending. The conference is to be held in a building on the Thames, and the Doctor concludes that the Skarasen will attack the conference. UNIT races to London, but Broton activates his signal device, which calls the monster. The Brigadier kills Broton and the Doctor recovers the device as the Skarasen surfaces. The Doctor throws the device down the Skarasen’s throat, and it submerges and swims out to sea. The group returns to Scotland. The Brigadier plans to cover up the incident. The Doctor offers to take them back to London in the TARDIS, but Harry and the Brigadier decline.
This marked the end of Harry Sullivan’s tenure as a companion, and the Brigadier did not appear again until 1983. The outdoor scenes were shot in West Sussex and other places because Scotland was too expensive. Few people noticed that Loch Ness without mountains was quite wrong. The Zygons, who looked like fish with ugly human faces and octopus-suckers all over, were praised by critics—the Loch Ness Monster not so much. Their ship technology was quite interesting, the bridge controls like a big pizza which you worked by twisting mushrooms in an obscene manner, but the Zygons themselves were rather pizza-like, so it kind of worked. I rather enjoyed this one, most of which was a well-paced atmospheric mystery. There was a lot of tension at the BBC at the time, since Sir Lew Grade was about to come out with Space: 1999, which everyone thought would blow Doctor Who out of the water. But as someone said, the production designed to appeal to Britons and Americans alike sank in the mid-Atlantic. Sir Lew had many triumphs in TV, but Space:1999 and UFO were not among them. None of them had the heart of Doctor Who.