Terror of the Zygons is the first serial of the thirteenth season of the British science fiction television series Doctor Who, which was the first broadcast in four weekly parts on BBC1 from 30 August to 20 September 1975. The serial featured the last regular appearance by Ian Marter as companion Harry Sullivan and Nicholas Courtney as Brigadier Lethbridge-Stewart.

This article's lead section may be too short to adequately summarize the key points. (July 2010)

The serial is set in and around Loch Ness and in London. In the serial, the alien shapeshifters the Zygons plot to use their cyborg sea monster the Skarasen to take over the Earth, after they discover their home planet was destroyed.

The Fourth Doctor, Sarah Jane Smith and Harry Sullivan arrive, via the TARDIS, in Scotland, near the North Sea, where Brigadier Lethbridge-Stewart and U.N.I.T. are investigating the destruction of oil rigs. The survivors' assertion that the rigs were destroyed by a huge sea creature is corroborated by giant tooth marks in the wreckage.

During their investigation, Harry is captured by the Zygons, a shapeshifting alien race hiding in their submerged spacecraft. Their leader, Broton, tells Harry that their spaceship had sustained damage and landed on Earth centuries ago to await rescue, but when they discovered that their home planet had been destroyed in a stellar explosion, they decided instead to conquer the Earth and terraform it to suit their physiology. To achieve this goal, they have captured several humans to use as "body prints" to infiltrate key leadership positions, including the influential Duke of Forgill who serves as head of the Scotland Energy Commission. They had also brought an embryonic sea creature called the Skarasen to Earth, and augmented it with cyborg technology until it has reached giant proportions. The Zygons are directing it with a signalling device to attack the rigs as part of their larger plan.

Whilst investigating the similarity between the Skarasen and the monster in nearby Loch Ness, Sarah Jane stumbles upon a secret passageway at the Duke of Forgill's mansion. She follows the passage to the Zygons' submerged spacecraft. Whilst searching the ship she locates and frees Harry, who reveals the Zygon stratagem.

With their presence discovered, Broton accelerates the Zygons' plan. He takes the Duke's form and leaves for London, while the remaining Zygons fly their ship to a nearby quarry. Once there, the Zygons use reactors to convert the Earth's atmosphere to one hospitable to Zygons, but poisonous to humans. The Doctor sneaks aboard the ship, frees the remaining humans and sets the ship to self-destruct, killing the Zygon crew.

Among the rescued humans, the Duke warns that he was scheduled to attend the first international energy conference in London that day, at which several high-level dignitaries will be in attendance. With the conference located in a building near the Thames, the Doctor fears that Broton will lure the Skarasen to attack the conference. U.N.I.T. races them to London but, before the Doctor can stop him, Broton activates the signalling device. The Brigadier kills Broton, and the Doctor recovers the device just as the Skarasen surfaces. The Doctor throws the device into the Thames, the creature eats it and, no longer a threat, returns to Loch Ness.

The group returns to Scotland to close up the investigation, and the Brigadier reports that the Cabinet will cover up the incident. The Doctor offers them all a return trip back to London via the TARDIS, but the Brigadier and Harry decline.

Interested in gaining new writers for Doctor Who, script editor Robert Holmes discussed ideas for the programme with Robert Banks Stewart. After their meeting in early 1974, Stewart devised a storyline for a six-part adventure called The Secret of Loch Ness. Stewart felt that Scotland's legendary Loch Ness Monster would make an ideal basis for a story because there were so few details about the mythical creature. Although at first focusing on the Loch Ness Monster itself, Holmes encouraged Stewart to concentrate more on the Zygons, the shapeshifting aliens of the story. As the story evolved, it was known variously as The Loch, The Secret of the Loch, The Loch Ness Monster, The Zygons and finally Terror of the Zygons.^{[citation needed]}

Terror of the Zygons was originally intended as the finale for Season 12, ending the TARDIS crew's continuous adventures and delivering Harry Sullivan back to Earth. Instead, due to scheduling concerns, the serial was held back as the first story of Season 13.

Due to budgetary constraints, location filming in Scotland was not possible and outdoor scenes for Terror of the Zygons were all shot in West Sussex. The North Sea beach scenes were filmed on the beach at Climping; scenes on Tulloch Moor were shot on the common at South Ambersham in the South Downs; and the landing area for the Zygon spaceship was filmed at Hall Aggregates Quarry in Storrington.^[1]

In The Television Companion (1998), David J. Howe and Stephen James Walker wrote that Terror of the Zygons gave a stereotypical portrayal of the Scottish and showed how much the show had changed since abandoning its regular UNIT premise. They felt that the story gave UNIT its "dignity and believability" and praised the conception of the Zygons, though they noted that the shapeshifting concept was not original. Despite classifying the Skarasen as the "major weakness", they wrote that "the story remains a strong one".^[7] In 2010, Mark Braxton of the Radio Times praised the "exquisitely horrible" design of the Zygons and the cliffhanger of the first episode where a Zygon attacks Sarah. He also was positive towards guest actor John Woodnutt and the incidental music, calling the whole production "a class act", aside from the Loch Ness Monster.^[8] DVD Talk's John Sinnott gave the story four-and-a-half out of five stars, praising the cast and the design of the Zygons.^[9]

Ian Berriman of SFX felt that it was "churlish" to criticise the Loch Ness Monster effect when the story "gets so much right, including first-class direction, pitch-perfect performances and a hauntingly eerie, folky score". He also was positive towards the design of the Zygons and their spaceship, though he found their scheme farfetched.^[10] Christopher Bahn, reviewing the story for The A.V. Club, described it as "fun" but noted that it could be formulaic instead of trying to be "ground-breaking"; he criticised the scene in the second episode in which Broton tells Harry everything about the Zygons, which did not leave much surprise for the later episodes. Nevertheless, he praised the cast, the action sequences, and the Zygons, which he described as "wonderfully surreal triumph of Doctor Who visual design", though otherwise they functioned as a typical monster-of-the-week.^[11]

Reviewing the serial in 1999, literary critic John Kenneth Muir acclaimed Terror of the Zygons as "a riveting and horrifying adventure", singling out the fleshy Zygon costumes for particular praise. He drew parallels with a number of historic Doctor Who serials, noting that the Zygon story drew on some familiar Doctor Who ingredients, including alien invasion (The Invasion), "body snatchers" (The Faceless Ones), an oil rig setting (Fury from the Deep), biomechanical technology (The Claws of Axos) and the revelation of an ancient Earth legend to be alien in origin (The Dæmons). However, he was disparaging of the use of a glove puppet to represent the Loch Ness Monster, comparing it to "the Invasion of the Dinosaurs debacle".^[12]