The Black Guardian, embodiment of evil and darkness, looms as the search for the final segment of the Key To Time brings the TARDIS to Atrios. There, the time travellers find the zealous Marshal determined to win a protracted war with the neighbouring world of Zeos, even as the beloved Princess Astra struggles to hold her people together. But Astra goes missing, while Romana discovers that the Marshal is being influenced by an unknown force. A journey to Zeos reveals a barren world governed only by the computer Mentalis. At the heart of the mystery is the sinister Shadow... and his own quest for the Key To Time.

When he developed the Key To Time story arc which would run through Doctor Who's sixteenth season, producer Graham Williams recognised that it would complicate the year's planning process. As such, he and script editor Anthony Read decided to place the climactic serial in experienced hands. They approached Doctor Who veterans Bob Baker and Dave Martin, who had most recently written both The Invisible Enemy and Underworld for Season Fifteen. Baker and Martin were given free rein to devise an appropriate conclusion to the umbrella theme, as long as they ensured that the Key To Time saga did not spill over into the subsequent year.

Baker and Martin took inspiration from the ongoing Cold War between the United States and the Soviet Union to compose an adventure entitled “Armageddon”. As part of their research, they visited the Central Government War Headquarters, a thirty-five-acre underground complex in Corsham, Wiltshire which was built in the late Fifties to provide a refuge from which the British government could continue to operate in the event of nuclear war. Although the facility was normally off limits to the public, Baker and Martin were delighted to learn that the officer in charge was a fan of Doctor Who, and he arranged their tour.

Originally, the Shadow's own shadow turned out to be the sixth segment of the Key To Time

In the original storyline for “Armageddon” -- submitted on December 19th, 1977 -- both Atrios and Zeos were populated. The conflict between them stemmed from a catastrophic shift in the orbits of both worlds, for which each blamed the other. They were being egged on by the Shadow, known as “the Presence” on Atrios and “the Voice” on Zeos. The Doctor helped Merak search for an astrophysicist named Reina, who had gone missing years earlier; it transpired that she had discovered the Shadow's planet lying between the two warring worlds. An elderly Zeon named Asgard provided clues to Reina's whereabouts. The Doctor was forced to use a jury-rigged Key To Time to keep both planets' armies frozen in time. The Shadow's own shadow turned out to be the sixth segment of the Key. His plan was to use its powers to pit one half of the universe against the other half. The Doctor stopped the Shadow by unfreezing the Atrian and Zeon armies, and giving each the coordinates of the Shadow's Castle of Evil.

A key supporting character in “Armageddon” would be Drax, a Time Lord mechanic whom Baker and Martin had created for their unused original version of 1976's The Hand Of Fear. They initially envisaged Drax as an elderly eccentric, thinking along the lines of the aged prisoner in Alexandre Dumas' novel The Count Of Monte Cristo (originally published in 1844 as Le comte de Monte-Cristo). Subsequently, Baker and Martin reimagined him as a character in the mould of a South London car salesman. One of the inspirations for this approach was Trevor Ray, who had been Doctor Who's assistant story editor in 1968 and 1969.

Baker and Martin were commissioned to write the scripts for “Armageddon” on January 27th, 1978. To avoid confusion with Prince Reynart from the same season's The Androids Of Tara, Reina became Astra; she was now a princess. It would be Astra, not the Shadow's shadow, who would be revealed as the sixth segment of the Key To Time. The notion of one of the segments being a person had previously been suggested by Douglas Adams, whose The Pirate Planet was another of the Key To Time serials.

Around the end of the summer, Baker and Martin decided to conclude their decade-long writing partnership. As such, “Armageddon” would become Martin's final Doctor Who serial, although Baker would provide 1979's Nightmare Of Eden on his own. The burden of rewrites consequently fell chiefly on Read's shoulders. Of particular concern was the production cost of “Armageddon” and, to this end, it was decided to eliminate the Zeons altogether, with Zeos being run entirely by the computer Mentalis. Read and Williams also played up Drax's comical nature, portraying him as more of a Cockney shyster. It had ultimately been agreed that Baker and Martin would leave the climax of the Key To Time storyline to Williams, but he was nonetheless influenced by their ideas. In particular, Baker and Martin had suggested that the Doctor should come to mistrust both the Black and White Guardians. Indeed, there was some discussion about the possibility of the two Guardians being different aspects of a single entity.

Douglas Adams worked with Graham Williams to add the Doctor's confrontation with the Black Guardian

By the middle of October, Read was being trailed by Adams, who was preparing to take over the script editor's post for Season Seventeen. It was Adams who changed the name of Baker and Martin's serial to The Armageddon Factor. He also worked with Williams to revise the story's conclusion by adding the Doctor's confrontation with the Black Guardian. As originally written, the Doctor simply decided that he did not want anyone controlling the Key To Time, whereupon he dispersed the six segments across time and space.

The Armageddon Factor was directed by Michael Hayes, who had just finished work on The Androids Of Tara. It was originally hoped that Cyril Luckham -- who had played the White Guardian in the season premiere, The Ribos Operation -- would appear as the Black Guardian in Episode Six, but he proved unavailable. Instead, Hayes recruited Valentine Dyall, who was best known for providing the sepulchral voice of the Man In Black for the BBC Radio horror anthology Appointment With Fear. As Princess Astra, he cast Lalla Ward; she immediately hit it off with the Doctor Who cast and crew, especially Tom Baker.

Rehearsals for The Armageddon Factor began amidst simmering tensions behind the scenes. Williams was resisting Baker's attempts to wrest greater creative control over Doctor Who, leading Baker to threaten his resignation from the programme, and the BBC to consider his dismissal. Towards the end of October, however, a détente was reached following a meeting between Baker and BBC Head of Drama Shaun Sutton, with the actor agreeing to rescind his resignation until further discussions could be held. Meanwhile, Williams also found himself uncertain as to the status of Mary Tamm, who was noncommittal about returning to Doctor Who for Season Seventeen.

Model filming for The Armageddon Factor took place at the BBC Television Film Studios in Ealing, London on October 27th. Production then moved to BBC Television Centre Studio 3 in White City, London for three three-day blocks. The first session spanned November 5th to 7th. The War Room was a key set in use on the first two days, alongside the propaganda segments on the 5th, and sequences in the black room on the 6th. The middle day also saw the start of work on material in K-Block, and this continued on the 7th alongside a variety of corridor scenes.

The second studio session ran from November 20th to 22nd. All three days experienced delays due to a new round of industrial action, which had similarly plagued The Invasion Of Time, the previous year's finale. Hayes started the block with sequences in the interrogation chamber and various corridors on Zeos. The next day, the Doctor's climactic confrontation with the Black Guardian was recorded on the TARDIS set, along with material in the Marshal's command module. Work on the 22nd concentrated on scenes in the chamber housing Mentalis, as well as more action in the corridors.

Tom Baker suggested a portly companion in the vein of comedienne Miriam Margolyes

In late November, Williams met with Tamm and her agent in an effort to clarify her status for Season Seventeen. Williams was eager to retain the actress for a second year, but she felt that the production team had allowed Romana to degenerate into just another screaming companion. Williams even intimated that he might be willing to part with Baker if it would keep Tamm on Doctor Who but, despite his efforts, he was still unable to secure her firm decision. Aware of the situation, Baker began campaigning for the next companion to deviate wildly from the norm, suggesting a bedraggled young urchin or a portly woman in the vein of comedienne Miriam Margolyes.

The final recording session for The Armageddon Factor took place from December 3rd to 5th. The block was principally concerned with scenes set on the Shadow's planet, although part of the first day was spent on material at Merak's hospital on Atrios, while the remaining TARDIS scenes were recorded on the last day. December 5th also marked the end of Doctor Who's sixteenth production block.

Williams opted to proceed as if Tamm would be remaining on the show. The actress herself, however, was now leaning towards leaving Doctor Who, and suggested that Ward might be suitable to take over as a new incarnation of Romana. This notion was greeted with delight by Baker, who had become very fond of Ward; indeed, Tamm had spent much of the production trying to keep her co-star away from the younger actress. Finally, in mid-December, Tamm confirmed to Williams that she would not be returning to Doctor Who.

Meanwhile, Williams was still trying to deal with Baker. The star had been offered a contract for Season Seventeen on December 15th. However, two weeks then elapsed without Baker's commitment, and he indicated that none would be forthcoming until he was assured of his involvement in casting the new companion. Williams was increasingly of the opinion that Baker had played the Doctor for too long, and he asked the actor's agent for a final decision on the contract renewal by January 4th, 1979.

This ultimatum was the final straw for Baker; he wrote to BBC Head of Serials Graeme MacDonald to reinstate his resignation, and to decry the way he was being treated by Williams. The producer, in turn, informed MacDonald that he would quit if Baker's demands were met. At last, MacDonald brought Baker and Williams together to clear the air. Williams remained resolute, while Baker apparently realised that his antics had gone too far. Following this meeting, Baker finally signed his contract on January 18th -- having been granted none of his demands for control over scripts, directors or casting.

The British press began reporting Mary Tamm's departure from Doctor Who on January 20th

Nonetheless, a few days later, Baker was delighted to learn that Tamm's suggestion had been taken up, and Ward was being cast as a regenerated Romana. This came after Williams had decided against developing a brand new character. He had also toyed with the idea of a different actress playing Romana in every serial -- a similar scheme having also been considered for Season Sixteen -- before deciding that the need to motivate each regeneration would place an unnecessary burden on the writers. The British press began reporting Tamm's departure from Doctor Who on January 20th. Ward was contracted for Season Seventeen on the 24th, and a photocall was held on February 6th.

Episode Six of The Armageddon Factor was broadcast on February 24th, bringing Doctor Who's sixteenth season to a close. It was scheduled for 6.30pm -- five minutes later than usual -- due to a slightly extended edition of Grandstand that afternoon. By now, Williams had finally brought some renewed stability to Doctor Who after months of uncertainty. Nonetheless, the strain of two difficult seasons had begun to take its toll on the producer, and Williams began considering whether it was time to move on from Doctor Who himself...

• Doctor Who Magazine #223, 15th March 1995, “Archive: The Armageddon Factor” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #9, 22nd December 2004, “I'll Put You Together Again” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #30, 2016, “Story 103: The Armageddon Factor”, 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 #37, April 1992, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.