Terror of the Zygons is the first serial of the thirteenth season of the British science fiction television series Doctor Who, which was first broadcast in four weekly parts on BBC1 from 30 August to 20 September 1975. The serial was written by Robert Banks Stewart and directed by Douglas Camfield.

The serial stars Tom Baker as the Fourth Doctor alongside Elizabeth Sladen and Ian Marter as companions Sarah Jane Smith and Harry Sullivan, with the episode featuring Marter's and Nicholas Courtney's (as Brigadier Lethbridge-Stewart) final regular appearances. The serial is the first appearance of the Zygons, and 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 serial received a novelisation written by Terrance Dicks, and several DVD releases. Terror of the Zygons was met with generally positive reviews. The episode's soundtrack, composed by Geoffrey Burgon, was released on 24 January 2000.

The Fourth Doctor, Sarah Jane Smith and Harry Sullivan arrive in Scotland, where Brigadier Lethbridge-Stewart and UNIT 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.

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 Skarasen and the Loch Ness Monster, Sarah Jane stumbles upon a secret passageway at the Duke of Forgill's mansion, which leads her to the Zygons' spacecraft. Whilst searching the ship, she locates and frees Harry, who reveals the Zygon stratagem. With their presence discovered, Broton accelerates the Zygons' plan. The Doctor sneaks aboard the ship, frees the remaining humans, and causes 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. Before the Doctor can stop him, Broton activates the signalling device but is killed by the Brigadier; the Doctor recovers the device just as the Skarasen surfaces. The Doctor throws the device into the Thames; the creature eats it and 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.

Terror of the Zygons was commissioned in April 1975 under the working title The Loch, changed later to The Secrets of the Loch. The serial was the first serial that Robert Banks Stewart wrote for the programme. He set the serial in his native country Scotland, and adapted elements of the Loch Ness Monster myth. Stewart subsequently wrote the finale of the season, The Seeds of Doom (1976). The serial is set 10 years after its release in 1985.

When Robert Holmes became head writer on the programme he begun to slowly phase out UNIT a process which he intended to complete at the end of season 12, which Terror of the Zygons was the intended finale for, by ending the TARDIS crew's continuous adventures and delivering Harry Sullivan back to Earth. However, Stewart could not complete the script in time so the serial was held back and became the first story of season 13, with a reduced order of four episodes instead of the original six. When Nicholas Courtney learned of Holmes' plan he suggested that his character should be killed off.

The episode was directed by Douglas Camfield. Camfield previously directed The Invasion (1967) which introduced UNIT; reportedly Camfield disliked the new costumes used for UNIT and preferred the ones used in The Invasion. Filming had to be rescheduled due to budget cuts. These cuts forced Courtney to exit a theater role. Location filming took place in March 1975. According to Tom Baker, due to rain the filming of the episode was delayed by two days. Baker also stated that due to budgetary constraints, location filming in Scotland was not possible. Instead, location filming for Terror of the Zygons was shot in West Sussex, including at Climping beach, South Ambersham in the South Downs, and at the Hall Aggregates Quarry in Storrington. Studio filming took place the following month.

John Woodnutt had previously appeared alongside Jon Pertwee's Third Doctor as Hibbert in Spearhead from Space and the Draconian Emperor in Frontier in Space. He would go on to play Consul Seron in the penultimate Fourth Doctor serial, The Keeper of Traken. Angus Lennie previously played Storr in The Ice Warriors. This was Nicolas Courtney's last regular appearance in the series. The Brigadier would next be seen in Mawdryn Undead, almost eight years later. The Zygons would not return until the 50th anniversary special, "The Day of the Doctor".

Episode	Title	Run time	Original air date	UK viewers (millions)	Appreciation Index
1	"Part One"	21:41	30 August 1975 (1975-08-30)	8.4	59
2	"Part Two"	25:08	6 September 1975 (1975-09-06)	6.1	—
3	"Part Three"	24:09	13 September 1975 (1975-09-13)	8.2	54
4	"Part Four"	25:22	20 September 1975 (1975-09-20)	7.2	—

Terror of the Zygons was released on BBC1 in four weekly parts from 30 August to 20 September 1975. "Part One" was released to an audience of 8.4 million viewers making it the highest viewed episode. It was followed by the lowest viewed, "Part Two", with only 6.1 million. Parts three and four were viewed by 8.2 and 7.2 million viewers respectively. Audience Appreciation Index were taken for the first and third episodes, they scored ratings of 59 and 54.

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". 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. 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.

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. 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 a "wonderfully surreal triumph of Doctor Who visual design", though otherwise they functioned as a typical monster-of-the-week.

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".

Doctor Who and the Loch Ness Monster

Author	Terrance Dicks
Cover artist	Chris Achilleos
Series	Doctor Who book: Target novelisations
Release number	40
Publisher	Target Books
Publication date	15 January 1976
ISBN	0-426-11041-2

A novelisation of this serial, written by Terrance Dicks, was published by Target Books in January 1976 under the title Doctor Who and the Loch Ness Monster.

Terror of the Zygons was first released in Australia on VHS in April 1987. It was later released in the United Kingdom on VHS in November 1988. It was first released in complete and unedited episodic format on LaserDisc in 1997. A new VHS release, also in episodic format, was released in August 1999 in the United Kingdom, and released in 2000 in the United States and Canada.

The serial was released on DVD on 30 September 2013. It features a director's cut version of Part One, with a previously unseen and newly restored 1 min 40-second opening scene featuring the Doctor, Sarah and Harry arriving in the TARDIS, which has materialised invisibly due to a faulty fusion plate. The restored scene was recoloured by Stuart Humphryes. The DVD release also included several promotional items including interviews, advertisements for the serial, and Bakers appearance on Disney Time.

A single-disc version (with no extras) of the DVD formed part of the Fourth Doctor Time Capsule, released on 29 July 2013.

Doctor Who: Terror of the Zygons
Soundtrack album by Geoffrey Burgon
Released	24 January 2000
Genre	Soundtrack
Length	78:25
Label	BBC Music
Producer	Mark Ayres
Doctor Who soundtrack chronology
Original Soundtrack Recording (1997) Terror of the Zygons (2000) Doctor Who at the BBC Radiophonic Workshop Volume 1: The Early Years 1963–1969 (2000)

Geoffrey Burgon's music for his Doctor Who serials Terror of the Zygons and The Seeds of Doom were released on CD by BBC Music on 24 January 2000. The CD was sourced from the composer's own copies of the score, recorded at a low speed, resulting in lower fidelity.

• Cornell, Paul; Day, Martin; Topping, Keith (1995). "Terror of the Zygons". The Discontinuity Guide. London: Virgin Books. pp. 179–180. ISBN 0-426-20442-5.
• Haining, Peter. Doctor Who: 25 Glorious Years W H Allen (1988) ISBN 1-85227-021-7
• Lofficier, Jean-Marc (1994). The Doctor Who Programme Guide (Third ed.). Target Books. ISBN 0-426-20342-9.
• Barns, Alan (21 July 2004). "Terror of the Zygons". Doctor Who Magazine. No. 345. Retrieved 22 July 2024.
• Ainsworth, John (2015). "Doctor Who - The Complete History: Genesis of the Daleks, Revenge of the Cybermen and Terror of the Zygons". Doctor Who - The Complete History. Vol. 23. Panini Comics. Retrieved 18 August 2024.

• Terror of the Zygons at BBC Online

• Doctor Who and the Loch Ness Monster title listing at the Internet Speculative Fiction Database