The Doctor returns to Gallifrey, having made a deal with aliens known as the Vardans to ensure that he will ascend to the presidency of the High Council of the Time Lords. Behaving erratically, the Doctor alienates his old friend, Chancellor Borusa, and forms an alliance with the duplicitous Castellan Kelner. Then he has Leela exiled to the wastes beyond the Capitol, where she takes refuge with a group of outcast Gallifreyans living as savages. Leela believes that the Doctor has turned traitor. In fact, he is orchestrating an elaborate scheme to unveil the identity of the Vardans' masters -- and foil a plot to invade Gallifrey itself.

When he joined Doctor Who midway through Season Fifteen, script editor Anthony Read became responsible for its last two serials. Read's first commission was Underworld, from the writing team of Bob Baker and Dave Martin. For the season finale, producer Graham Williams wanted to delve deeper into the vision of Gallifreyan society and mythology that Read's predecessor, Robert Holmes, had developed for The Deadly Assassin a year earlier. After discussing this approach with Holmes, the production team received his blessing to use Cardinal Borusa, one of that serial's key Time Lord characters.

To write the six-part finale, Read recruited David Weir, who had written several episodes of The Troubleshooters and The Lotus Eaters when Read was their producer. The two men began developing a narrative which drew upon the notion that not all of Gallifrey's denizens were Time Lords. On July 18th, 1977, Weir was commissioned to write “Killers Of The Dark”. It introduced a race of cat people living outside the Time Lord Capitol, whose society was defined by both brutal savagery and technological sophistication. Weir had to write quickly to meet the serial's mid-October production dates, and submitted his first five scripts on August 15th.

Killers Of The Dark included unfeasible elements such as a forum the size of Wembley Stadium full of cat people

By this time, Gerald Blake had been assigned to direct “Killers Of The Dark”; he had previously been responsible for 1967's The Abominable Snowmen. Both Blake and Read quickly became concerned about the feasibility of Weir's scripts, which had diverged significantly from their original outline. The Doctor Who production team was under strict instructions from the BBC to control the programme's budget -- a mounting challenge as inflation rates skyrocketed -- and yet Weir had included elements such as a forum the size of Wembley Stadium full of cat people. Left with little choice, Williams and Read decided to abandon “Killers Of The Dark”. This meant that they had to hastily commission six new scripts, at a point in time when pre-production should already have started; indeed, location scouting and design work for Weir's serial was under way.

To complicate matters further, it was clear that the BBC was heading towards another round of industrial action in the autumn. During the Seventies, the Corporation was pitted against its unionised employees on an almost annual basis, and 1977 was no different. With Christmas programming having been given the highest priority, it became clear that the Doctor Who season finale could be guaranteed only a single studio block, two fewer than would normally be allocated to a six-part story. Given the enormous pressure that was being placed on the production team, BBC brass suggested that Season Fifteen could simply be abbreviated, dropping the final serial altogether and ending with Underworld. To help alleviate financial pressures on Season Sixteen, the budget for the unmade adventure could then be carried over to the following year.

However, Williams and Read were determined to complete the full slate of episodes, and agreed to stick with the plan of developing a sequel to The Deadly Assassin. This was a decision born partly out of pragmatism: money could be saved by reusing sets and costumes from the earlier production. They would have to tread carefully, however, because they would now be required to avoid any story elements which could be deemed as having been derived from “Killers Of The Dark”. Williams and Read approached Holmes to see if he would write the serial; the former script editor declined because he wanted to put some space between himself and Doctor Who, although he did indicate that he might be interested in a commission for Season Sixteen. With time running out, by August 25th Williams and Read had concluded that they would have to write the story themselves.

Working together over a long weekend, Williams and Read developed a storyline entitled The Invasion Of Time. (“The Invaders Of Time” may also have been considered.) They contacted Holmes for advice, and he explained that he had constructed six-part serials such as The Seeds Of Doom and The Talons Of Weng-Chiang as the marriage of one four-part and one two-part story. He also gave Williams and Read permission to use the Sontarans, an alien race which Holmes had created for 1973's The Time Warrior and which were a favourite of Williams'. It was decided to save the Sontarans for the final two episodes, shocking the viewers who, over the course of the first four installments, would come to believe that the villains of the piece were new aliens called the Vardans.

Exploring the vast geography of the TARDIS was an idea Graham Williams had been nurturing for Season 16

To address the problem of the two cancelled studio blocks, it was agreed that Doctor Who would take advantage of an emergency fund, which would instead provide two weeks' worth of recording on Outside Broadcast (OB) videotape. However, this meant that Williams and Read would have to structure their storyline to suit existing locations. As a result, Williams decided to make use of an idea he had been nurturing for Season Sixteen. Large swathes of The Invasion Of Time would be set within the vast geography of the TARDIS, of which very little had previously been depicted.

With the storyline in place, Read began preparing the six scripts, and completed his initial drafts in about two weeks. With little time to make substantial refinements to the narrative, The Invasion Of Time underwent only a few changes. Originally, it was revealed that the Time Lords were not native Gallifreyans, but rather had been permitted to construct their Citadel there in return for ensuring the comfort and security of the planet's populace. It was these indigenous Gallifreyans who Leela encountered after her banishment. The Doctor's actions as President threw the Time Lords into open civil war, and Episode Two ended with several rebellious Time Lords threatening to execute him. This cliffhanger scenario was ultimately shifted to Episode Three, with Commander Andred alone confronting the Doctor. The earlier version of Episode Three closed with the Vardans betraying the Doctor and ordering his annihilation.

Williams then spent a week script editing The Invasion Of Time, and found himself contributing a substantial amount of material to the final two installments. He and Read felt that it was appropriate for the serial to be jointly attributed, but the BBC was reluctant for both a producer and a script editor to be seen writing for their own programme. Consequently, it was decided that The Invasion Of Time would be transmitted under a pseudonym. Williams suggested the alias “Richard Thomas”, after his young son, but Head of Serials Graeme MacDonald requested that “David Agnew” receive the credit. This was a nom de plume used on a variety of BBC programmes dating back to 1971.

A final wrinkle confronting The Invasion Of Time was the matter of the Doctor's companions. The K·9 prop had posed problems ever since its introduction in The Invisible Enemy at the start of the production block. As such, it was agreed that the character would be written out at the season's end, with some consideration given to parting ways with K·9 altogether. However, it was soon decided that his departure would provide an excuse to introduce an improved version of the prop, and the final scene of The Invasion Of Time was written to reassure viewers that K·9 “Mark II” would debut the following season. John Leeson was contracted to provide K·9's voice for all but the fifth episode of The Invasion Of Time on October 12th.

Graham Williams was optimistic that he could change Louise Jameson's mind about returning for Season 16

Less easily dealt with was Leela. Williams was enthusiastic about the character and, despite Louise Jameson's insistence that she was not interested in returning for Season Sixteen, Williams remained optimistic that he could change her mind. Although he had started devising a new companion -- a Time Lord called Romanadvoratrelundar or “Romy” -- he gave little thought to Leela's departure in The Invasion Of Time because he was anticipating a last-minute rewrite to retain her. As a result, Leela's abrupt decision to remain on Gallifrey with Andred came as an enormous disappointment to Jameson, who would have preferred to see her character killed off. Williams and Read had actually contemplated this -- a development also mooted for The Sun Makers earlier in the year -- only to conclude that it would be too traumatic for the series' younger viewers, and a pessimistic way to end the season. In rehearsals, Jameson and Andred actor Chris Tranchell tried to introduce additional moments of tenderness between their characters, to make Leela's exit more credible.

The production team had hoped to maintain continuity with The Deadly Assassin by engaging the services of the same designer, Roger Murray-Leach. However, he was unavailable for the production dates, and so Barbara Gosnold took his place. Similarly, Angus Mackay was unable to return to play Borusa, which led to John Arnatt being cast as the Time Lord's latest regeneration. (Indeed, across a total of four appearances in Doctor Who, Borusa would never be played by the same actor twice!) Meanwhile, Blake had to find a suitable actor to play the main Sontaran, Commander Stor. Kevin Lindsay, who had played various Sontarans in their previous appearances, had now passed away, and so Derek Deadman was hired instead. Williams was unhappy with Deadman's choice of accent for the role, and the actor refused to heed the producer's request to modify it.

Recording for The Invasion Of Time began with model filming at Bray Studios in Water Oakley, Berkshire on November 1st and 2nd. As with Underworld, Williams was keen for the model shots to be of notably high quality, to try to minimise any unfavourable comparisons between Doctor Who and Star Wars, which would be opening in British cinemas around the time that The Invasion Of Time arrived on television screens.

The lone studio block followed, spanning November 6th to 8th and taking place at BBC Television Centre Studio 8 in White City, London. The first two days concentrated on material in the TARDIS control room and the adjacent corridor. Blake also started recording on the Panopticon set on the 7th; this work continued to the last studio day, when material in the chamber where the TARDIS materialised and in the Vardan war room was also completed. Williams was still trying to persuade Jameson to return to Doctor Who right up to the studio recording, but now learned that she had committed to stage work which would overlap with the Season Sixteen production schedule. As such, Jameson's forthcoming departure was announced to the press on November 7th.

A week of location filming followed, starting with four days in Redhill, Surrey. On November 14th and 15th, Beachfields Quarry was used for scenes in Outer Gallifrey. Blake's team then moved to St Anne's Hospital for the afternoon of the 15th, with work there continuing on the 16th and 17th. A former nursing home, St Anne's was now made available for use in television productions, and featured space large enough to erect sets. Material in the Panopticon control room was recorded on the 15th and part of the 16th, with the rest of the time at St Anne's devoted to various sequences set in the labyrinthine interior of the TARDIS, including the sick bay. Cast and crew returned to London on November 18th, where the basement of the British Oxygen Building in Hammersmith offered a pool area suitable for the TARDIS “bathroom”.

Work resumed at St Anne's a fortnight later, with the footage now captured on OB videotape rather than film. In the meantime, on November 25th, Tom Baker was contracted for Season Sixteen. December 5th was spent on scenes in the TARDIS workshop, art gallery and power station. Recording on the 6th included material in space traffic control, Leela's room, the empty presidential office and Borusa's office. Work on the latter continued on the 7th, when Blake also started on sequences in Kelner's office. The Castellan's quarters remained the focus on December 8th and 9th, although the latter also saw more scenes in the Chancellery completed, and the start of taping in the Doctor's furnished office.

After the weekend, cast and crew returned to St Anne's on December 12th. The first order of business was the remaining scenes in the President's Office, as well as those in the Conservatory. The giant plant which consumed the Sontaran trooper inspired Tom Baker to suggest that his next companion should be a talking cabbage, to whom he could explain the plot as it perched on the Doctor's shoulder. The rest of the week, through to December 16th, was spent on various corridor sequences. The conclusion of work on The Invasion Of Time marked the end of both Doctor Who's fifteenth production block and Louise Jameson's tenure on the show. In turn, the broadcast of Episode Six on March 11th, 1978 drew Season Fifteen to a close.

• Doctor Who Magazine Winter Special 1992, “Archive: The Invasion Of Time” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #8, 1st September 2004, “Nobody Does It Better” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #28, 2017, “Story 97: The Invasion Of Time”, edited by John Ainsworth, 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 #29, January 1991, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.