The Doctor returns to Gallifrey, having become President of the High Council following an illicit deal with aliens known as the Vardans. He has Leela exiled to the wastes beyond the Capitol, where she allies herself with outcast Time Lords living as savages. Leela believes the Doctor has turned traitor, but in fact he is masterminding an elaborate plan to unveil the identity of the Vardans' masters, and foil a scheme to invade Gallifrey itself.

When he joined Doctor Who midway through its fifteenth season, new 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. Thought was given to making this the finale, but it was ultimately assigned to the penultimate slot. Read's predecessor, Robert Holmes, was also approached about writing the remaining story. Producer Graham Williams had been impressed by the vision of Time Lord society and mythology that Holmes had developed in the previous season's The Deadly Assassin, and thought it ripe for further exploration. However, Holmes did not want to return to Doctor Who so quickly -- although he was open to a contribution for Season Sixteen -- and declined the offer. He did indicate, however, that the production team should feel free to use Cardinal Borusa, a character he had introduced in The Deadly Assassin.

Read next turned to David Weir, with whom he had worked on The Troubleshooters. On July 18th, 1977, Weir was commissioned to write the six-part “Killers Of The Dark”, an ambitious adventure about a race of cat people with ties to Gallifrey. Weir began submitting his scripts to the production office about a month later, by which time Gerald Blake had been brought on board to direct Serial 4Z. This was Blake's second and final Doctor Who outing, having previously helmed The Abominable Snowmen a decade earlier. Blake went on to direct episodes of programmes such as Blake's 7, Play For Today and Super Gran before his death on April 5th, 1991.

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

Blake quickly became concerned about the feasibility of Weir's scripts for “Killers Of The Dark”. The production team was under strict instructions from the BBC to control the programme's budget -- an especially gruelling task given the mounting inflation rates of the time -- and yet Weir had included elements such as a forum the size of Wembley Stadium full of cat people. Read concurred with Blake's judgement, and “Killers Of The Dark” was hastily abandoned. With costs also proving to be a concern on Underworld, BBC brass suggested that Season Fifteen simply be truncated to twenty episodes, dropping the final serial altogether. However, Williams and Read were determined to complete the full slate of episodes, and concluded that the only way to proceed was to write the season finale themselves.

Sticking closely to Williams' original desire to make a sequel to The Deadly Assassin, the pair concocted a story entitled The Invasion Of Time (“The Invaders Of Time” may also have been considered). By setting the adventure on Gallifrey, money could be saved by reusing sets and costumes from the earlier production. They were assisted by Holmes, who advised them to construct the serial as interrelated four-part and two-part stories, a tactic he had found worked well on adventures such as The Seeds Of Doom and The Talons Of Weng-Chiang. He also gave Williams and Read permission to use the Sontarans, which Holmes had created for The Time Warrior in Season Eleven and were a favoured monster of Williams'.

Williams and Read faced another challenge in devising The Invasion Of Time. During the Seventies, the BBC was confronted with virtually annual industrial disputes with its unionised employees, and 1977 was no different. With studio schedules cast into disarray and Christmas programming deemed to have the highest priority, it became clear that Serial 4Z could be guaranteed only a single studio block, rather than the three sessions normally allocated to a six-episode story. Fortunately, an emergency fund existed which would provide Doctor Who with two weeks' worth of Outside Broadcast (OB) taping. This meant, however, that Williams and Read had to structure their storyline to take advantage of existing locations. As a result, they decided to set large swathes of the adventure within the vast geography of the TARDIS, of which little had been seen over the course of the programme's history.

Originally, the Doctor's actions as President threw the Time Lords into open civil war

On August 25th, permission was granted for Read to write the six scripts of The Invasion Of Time from the storyline conceived by himself and Williams. Owing to the great pressure that Read was under, the storyline underwent only a few changes. Initially, 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 that the planet's populace lived in total comfort and security. It was these indigenous Gallifreyans that Leela met following her banishment. The Doctor's actions as President originally threw the Time Lords into open civil war, and part two ended with several rebellious Time Lords threatening to execute the Doctor (much as Andred does in the part three cliffhanger of the finished serial). The Vardans were also to betray the Doctor, ordering his annihilation at the close of episode three.

As Read penned The Invasion Of Time, Williams served as his script editor, and also found himself contributing a substantial amount of material to the final two installments. They felt that it was not appropriate for Read to receive the sole credit for writing Serial 4Z, but the BBC was not eager for both a producer and a script editor to be seen writing for their own programme. Consequently, it was agreed 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 McDonald requested that it be credited to “David Agnew”, 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 did not function well, and so it was agreed that the character would be written out at the season's end. For a brief time, consideration was given to parting ways with K·9 altogether, but it was soon decided that his departure would actually 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 thought he could change Louise Jameson's mind about leaving, and so put little effort into Leela's departure

Less easily dealt with was Leela. Williams was enthusiastic about the character, and despite Louise Jameson's assurances 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 put little effort into Leela's departure in The Invasion Of Time because he was anticipating a last-minute rewrite to retain her. This did not come to pass, and so Leela's exit by means of an abrupt romance with Chancellery Guard 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), but concluded that it would be too traumatic for the series' younger viewers, and a pessimistic way to end the season.

Work on Serial 4Z began with two days of model filming at Bray Studios, on November 1st and 2nd. The lone studio block then followed from November 6th to 8th, in Television Centre Studio 8. This covered material in the TARDIS console room and the adjacent corridor, the Panopticon and the area where the TARDIS materialises, and the Vardan war room. Angus Mackay, who had played Borusa in The Deadly Assassin, was unavailable for The Invasion Of Time and so the role now went to John Arnatt -- beginning a tradition of Borusa never being played by the same actor twice. Meanwhile, to make up for Leela's bizarre exit from the programme, Jameson and Chris Tranchell (who portrayed Andred) agreed to contrive additional moments of tenderness between their characters during recording.

Location filming then began at Beachfields Quarry in Redhill, Surrey for the scenes in Outer Gallifrey. Recording there started on November 14th and continued into the next day, when the venue shifted to nearby St Anne's Hospital, which was now owned by the BBC and used for television productions. This meant that certain sets -- such as the TARDIS console room -- could be transported to and erected at St Anne's, essentially providing The Invasion Of Time with extra studio days. Shooting at the hospital occupied the remainder of November 15th through to the 17th, and dealt with sequences in the TARDIS and its various inner rooms and corridors, plus the Panopticon control room. Some of the material completed on the 16th was recorded on OB videotape to make effects work easier.

December 16th marked the conclusion of Louise Jameson's tenure on Doctor Who

The venue next shifted to the basement of the British Oxygen Building in London, for the scenes in and around the TARDIS “bathroom” on November 18th. After a fortnight's rest, work resumed with ten further days at St Anne's, spanning December 5th to 9th and 12th to 16th; OB recording was now used throughout. Work on these days covered the remaining TARDIS scenes, as well as those in the Time Lord Citadel -- the Offices of the President, Chancellor and Castellan, plus Space Traffic Control. As with Underworld, The Invasion Of Time was also allocated gallery-only days to finish the effects work and other post-production requirements. This would now become the norm for Doctor Who in future seasons.

December 16th marked both the end of Doctor Who's fifteenth recording block and the conclusion of Louise Jameson's tenure on the programme. She would go on to star in numerous stage and television productions, including The Omega Factor, Tenko, Bergerac and EastEnders, and would also develop a career teaching drama. Jameson returned to Doctor Who for the thirtieth-anniversary charity special Dimensions In Time in 1993. She also reprised the character for several audio dramas from Big Finish Productions, beginning with Zagreus in 2003 and encompassing the long-running Gallifrey series. Jameson's final regular Doctor Who episode, part six of The Invasion Of Time, brought the sometimes tumultuous Season Fifteen to a close on March 11th, 1978.

• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
• 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.
• In-Vision #29, January 1991, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.