The Brigadier summons the Doctor back to Earth to investigate the destruction of several oil rigs in the North Sea. UNIT has established a base of operations at Tulloch, near Loch Ness in Scotland. There, the Doctor discovers that the rigs were attacked by an enormous beast, while Harry is abducted after locating a survivor from the most recent incident. Taken to a spaceship crewed by the Zygons, Harry learns that they are preparing to take over the Earth using a cybernetic sea monster called the Skarasen. And because the Zygons are shapeshifters, the Doctor and his companions can trust nobody... not even each other.

When he became Doctor Who's script editor, Robert Holmes inherited several serials which had been initiated by the outgoing production team of Barry Letts and Terrance Dicks. However, he also began to recruit writers new to the programme, with the intention of cultivating a different narrative style. One candidate was Robert Banks Stewart, who met with Holmes in early 1974. Stewart indicated that he had no interest in an adventure set in outer space, but was keen to develop a storyline which took place in his native Scotland -- and, in particular, one which involved the legendary Loch Ness Monster.

Belief that Loch Ness was home to some kind of beast -- possibly prehistoric in origin -- may date from as long ago as the middle of the first millennium. The myth gained newfound popularity when a road passing by the loch was constructed in 1933, and interest reached an international scale during the Sixties. Stewart felt that the lack of established detail regarding the Monster's nature meant that it was particularly well-suited for a Doctor Who adventure. On March 12th, Holmes commissioned him to prepare a storyline under the title “Loch Ness”. A request for scripts followed on the 27th, with the serial now fixed at six episodes.

A key requirement of the scripts was that they would bring Harry Sullivan's travels in the TARDIS to an end

A key requirement of Stewart's scripts was that they would bring Harry Sullivan's travels in the TARDIS to an end. The character had been created at a time when it was thought that the Fourth Doctor might be played by an elderly actor who would be unable to participate in action sequences; with the casting of Tom Baker in the lead role, new producer Philip Hinchcliffe felt that Harry had been made redundant. Holmes disagreed, and argued that Harry was a valuable and distinctive ingredient of the series. The script editor ultimately acceded to his producer's wishes, but Hinchcliffe would later admit that Holmes' assessment had been correct.

Although it was intended that Harry would continue to make occasional appearances in Doctor Who, his exit also helped move the programme further away from the Earthbound format -- centred upon the United Nations Intelligence Taskforce (UNIT) -- which had propelled it through the early Seventies. Hinchcliffe wanted to set more adventures on alien worlds, and actors Nicholas Courtney and John Levene, who were contracted for “Loch Ness” to reprise their roles as Brigadier Lethbridge-Stewart and RSM Benton, were both aware of their diminishing presence in the series. Meeting with Hinchcliffe in the BBC bar, Courtney suggested that the Brigadier should be killed off, exiting Doctor Who in a blaze of glory. Hinchcliffe dismissed the notion, deeming it an inappropriate fate for a character who had been so vital to the programme for so long.

Stewart's progress on his scripts was slow, and lingered into the early part of 1975. During this period, the adventure variously went by the titles “The Loch Ness Monster”, “The Secret Of The Loch”, “Secret Of Loch Ness”, and “The Secret Of Loch Ness”. Unfamiliar with the style of Doctor Who, Stewart wrote the Doctor, Sarah and Harry in the vein of the protagonists of The Avengers, and had to be guided by Holmes in refining these elements. The script editor also encouraged Stewart to focus less on the Skarasen -- the putative Loch Ness Monster -- and more on the shape-shifting Zygons, whom he felt to be more interesting. The humanoid aliens also had the advantage of being able to interact directly with the main characters.

Meanwhile, several production decisions were being made which would impact “The Secret Of Loch Ness”. Throughout 1974, buzz had been building about Gerry and Sylvia Anderson's new science-fiction series Space: 1999, which was being developed for the rival ITV network. It became apparent that Space: 1999 was set for a September 1975 debut, and would run until the spring of 1976. Furthermore, it would potentially air on Saturday evenings, just like Doctor Who. Should Space: 1999 prove to be the massive hit that was being forecast, this would give it a four-month jump on the premiere date for the next season of Doctor Who which, since 1970, had fallen in late December or early January.

Instead of serving as the twelfth-season finale, Terror Of The Zygons would be held over to start Season 13

To maintain Doctor Who's visibility, and potentially blunt the ratings for Space: 1999, it was decided around January 1975 that Season Thirteen would be brought forward to start in the late summer or early autumn, as had been the case throughout most of the Sixties. This meant that the thirteenth recording block, which wouldn't normally start until September, would have to begin much earlier. In fact, it was determined that it should essentially follow on directly from the end of the twelfth recording block, for which “The Secret Of Loch Ness” was the final serial. Furthermore, it was agreed that Season Twelve should be trimmed from twenty-six episodes to only twenty -- the shortest in Doctor Who's history to that point. Instead of serving as the twelfth-season finale, “The Secret Of Loch Ness” would be held over to start Season Thirteen. But, with Hinchcliffe and Holmes eager to diminish the presence of six-part stories in the Doctor Who schedule, it would also be trimmed to just four episodes.

Holmes worked closely with Stewart to compress his narrative to suit the shorter length. A significant amount of revision would have been necessary anyway, since it was now known that the budget could not accommodate location filming in Scotland. As such, the importance of Loch Ness itself was greatly diminished. For example, the original cliffhanger for Episode Two involved the Skarasen attacking the Doctor as he rowed upon its waters, rather than traversing Tulloch Moor. Other changes saw Sister White become Sister Lamont, Stansgate House renamed Stansbridge House, and Claymore Oil rechristened Hibernian Oil.

“The Secret Of Loch Ness” was assigned to director Douglas Camfield, making his return to Doctor Who after a five-year absence. In 1970, Camfield had collapsed during the making of Inferno, when the stress of production aggravated a heart condition. He had been on a self-enforced sabbatical from the programme ever since, but was lured back by the premise of Stewart's story. Camfield may have contributed to the scripts' final form. The director also influenced the vision of the Zygons which was developed by costume designer James Acheson. While Stewart had simply suggested scaly creatures suitable for a maritime environment, Acheson drew upon the notion of the Zygons subsisting on the Skarasen's lactic fluid. The resulting costumes, intended to resemble oversized embryos, were a collaboration with visual effects designer John Friedlander.

Model work for “The Secret Of Loch Ness” was extensive, and involved both stop-motion animation and puppetry to bring the Skarasen to life. Unfortunately, Camfield found that, when moving, the monster appeared more comical than threatening, and so he decided to use as little of this footage as possible. Instead, new material was written for Episode Two which explained that the Zygons had deployed nerve gas on the denizens of Tulloch to allow the Skarasen to pass by unobserved.

By the time location filming began in West Sussex, the serial had been renamed “The Zygons” and finally Terror Of The Zygons, more accurately reflecting its emphasis. The principal location for the first three days -- March 17th to 19th -- was Ambersham Common in South Ambersham. This was where the TARDIS arrived, and was also the venue for scenes on the roadway, in the woods and on Tulloch Moor. The first part of March 17th, however, saw Munro washed ashore at Climping Beach in Climping, while work on the 18th began at Hall Aggregates Quarry in Storrington, for scenes outside the hospital and at the Zygon spaceship's landing area. On March 20th, the village of Charlton posed as Tulloch, and a barn in the community was the site of the ersatz Harry's attack on Sarah. The depth charge explosions were recorded the same day at Chichester. Finally, on the 21st, Furnace Pond in Crabtree posed as Loch Ness and its vicinity.

This should have marked the end of recording in West Sussex, but poor weather earlier in the week forced the allocation of two extra days. March 24th saw a return to Hall Aggregates Quarry to complete the material outside the hospital, while additional scenes on Tulloch Moor were recorded at Ambersham Common on the 25th. Finally, on March 26th, the London footage was apparently captured at Millbank Tower in Millbank. During the location shoot, the services of the remaining regular castmembers were secured for Season Thirteen, with Tom Baker contracted on March 21st and Elisabeth Sladen on the 25th.

The first studio session then took place in BBC Television Centre Studio 3 at White City, London. It occurred on a Monday and a Tuesday, as had been standard during the twelfth production block. April 7th was dedicated to Episode One, and April 8th to Episode Two. An extra day was added to the usual fortnight preceding the second studio block; it was held on Tuesday the 22nd and Wednesday the 23rd, this time in TC4. Episode Three was the main concern on the Tuesday, although Camfield also recorded the sequence of Harry and Broton watching the Skarasen for Episode Two, as well as the lone Forgill Castle scene from the concluding installment. The photocaption used for the establishing shot of the Duke's residence actually depicted Dunvegan Castle on the Isle of Skye.

Nicholas Courtney's next appearance in Doctor Who would not be until 1983

The rest of Episode Four was then taped on the Wednesday. Here, Nicholas Courtney ad-libbed the reference to the Prime Minister as “Madam”; knowing that the UNIT stories were intended to be set a few years in the future, Courtney's thought was that this might be Shirley Williams, a cabinet minister in Harold Wilson's recently-elected Labour government. For Courtney, Terror Of The Zygons would be his final appearance in Doctor Who for many years, concluding a regular association with the programme which dated back to the start of 1968. Although it had been intended that the Brigadier would be seen again later in Season Thirteen, Courtney's availability and changes to the planned storylines precluded this. Instead, his next appearance in Doctor Who would not be until Mawdryn Undead in 1983.

In post-production, Camfield was forced to make a significant edit to the start of Episode One. Originally, the TARDIS materialised in the woods near Tulloch and, when the time travellers emerged, they realised that a glitch had rendered the vessel invisible. The effect was accomplished by recording the two halves of the frame separately but, unfortunately, the light had changed so much between shots that the illusion was spoiled.

The first episode of Terror Of The Zygons aired on August 30th, marking the start of Doctor Who's thirteenth broadcast season. It was now scheduled at 5.45pm, slightly later than had been the case during Season Twelve. During the summer months, Doctor Who had generally been replaced by the variety programme Sing A Song Of Emu and, later, the magic show For My Next Trick. Doctor Who was now preceded by Walt Disney's The Mouse Factory and a news update. Episode One was followed by Seaside Special, and the middle two installments by Cliff Richards in It's Cliff -- And Friends. However, Episode Four on September 20th was brought forward to 5.20pm to accommodate the transmission of The Last Night Of The Proms later that evening. As such, Doctor Who came after Grandstand and the news, and before a special edition of Bruce Forsyth And The Generation Game.

• Doctor Who Magazine #210, 16th March 1994, “Archive: Terror Of The Zygons” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “You Ain't Seen Nothing Yet” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #23, 2016, “Story 80: Terror Of The Zygons”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• In·Vision #6, August 1988, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.