The Doctor decides to travel to Logopolis -- where pure mathematics can be used to reshape reality -- in order to repair the TARDIS and reactivate its chameleon circuit. First he goes to Earth to locate a real police box, and finds himself observed by a mysterious Watcher. Airline stewardess Tegan Jovanka stumbles into the TARDIS, unaware that her Aunt Vanessa has become the Master's latest victim. She travels to Logopolis with the Doctor and Adric, and there they are met by Nyssa, who is searching for her missing father. But it soon becomes clear that Logopolis itself is another part of the Master's apocalyptic trap...

On November 2nd, 1979, shortly after John Nathan-Turner's appointment as the new producer of Doctor Who, he and executive producer Barry Letts were approached by Head of Serials Graeme MacDonald about a new funding initiative for the series. Doctor Who had been a popular part of the Australian Broadcasting Commission's line-up for many years, and MacDonald suggested the possibility of a co-production arrangement. Keen to find ways to boost his show's budget, Nathan-Turner began negotiations with the ABC that lasted well into 1980, by which time it was too late for an agreement to impact Doctor Who's upcoming eighteenth season. Instead, the discussions centred on filming one or two serials in Australia for Season Nineteen -- but, by the end of July, an impasse appeared to have been reached.

Nathan-Turner was not yet inclined to give up on the idea, and he began casting about for ways to make Doctor Who more attractive to the ABC. At this point, he and script editor Christopher H Bidmead were developing a new female companion to replace Romana, who would be written out of the show midway through Season Eighteen. An earlier plan to bring back a popular face from the past had already been abandoned when both Elisabeth Sladen, who had played Sarah Jane Smith, and Leela actress Louise Jameson had declined Nathan-Turner's invitation to return to Doctor Who. Nathan-Turner now suggested that the new companion should be Australian, with the hope that the inclusion of an Antipodean regular would make the ABC's potential investment in Doctor Who more appealing.

Christopher H Bidmead misinterpreted the notation “Tegan (Jovanka)” as the new companion's full name

Nathan-Turner and Bidmead envisaged the new companion as a bolshy air hostess who would be willing to stand up to the Doctor, but whose forthrightness masked her lack of self-confidence. Nathan-Turner suggested two names for the character: Tegan (a Celtic name which Nathan-Turner selected as an homage to the niece of an Australian friend) and Jovanka (inspired by Jovanka Broz, the wife of Yugoslav President Josip Broz Tito). Bidmead inadvertently interpreted the notation “Tegan (Jovanka)” as the new companion's full name, and the misunderstanding stuck; a character outline for Tegan Jovanka was issued on August 1st. Unfortunately, by the end of August, the ABC had confirmed that they were still not interested in a Doctor Who co-production deal.

By late summer, it was established that not only would Tegan be introduced in the final story of Season Eighteen, but that this would also be Tom Baker's last Doctor Who adventure after seven years in the title role. Nathan-Turner and Bidmead had decided to bridge the transition from Baker to his successor with a trilogy of stories which resurrected the Doctor's Time Lord arch-nemesis, the Master. It would begin with The Keeper Of Traken, the year's penultimate serial, and conclude with the Fifth Doctor's debut at the start of Season Nineteen. Given the tremendous difficulty he had encountered in seeking new writers who were suitable for Doctor Who, Bidmead reluctantly agreed to provide the season finale himself. Staff clearance was secured on August 29th.

In addition to introducing Tegan, regenerating the Doctor, and spotlighting the Master, Bidmead decided to use his scripts to tie up loose threads from earlier in the year. The three middle stories of Season Eighteen had all dealt with the Doctor's adventures in the pocket universe of E-Space, into which the TARDIS was drawn after passing through a Charged Vacuum Emboitment. Bidmead decided to further explore this phenomenon in a narrative which came to be called Logopolis. The title was taken from the Greek term for “city of words”. Similarly, the Pharos Project was named for the Egyptian island on which once stood the Lighthouse of Alexandria, one of the Seven Wonders of the Ancient World.

A computer enthusiast, Bidmead took inspiration from ideas in computer science as he developed his scripts. Terminology related to computer architectures yielded elements such as the Monitor, block transfers, and the registers. Eager to inject Doctor Who with real scientific notions, Bidmead also drew upon the discipline of thermodynamics, making heavy use of the concept of entropy: the measure of unavailable energy in a system, which effectively increases as a system becomes homogeneous. Another starting point was Nathan-Turner's observation that a real police box still stood on the Barnet bypass in London. Bidmead was fascinated with the TARDIS, and wanted to explore its properties in greater detail.

The Master's on-screen appearance was delayed until Episode 3 to make the audience wonder if he might be the Watcher

As Bidmead worked on Logopolis, Nathan-Turner informed him that he was considering retaining Nyssa -- whom writer Johnny Byrne had introduced in The Keeper Of Traken -- as a third companion alongside Tegan and Adric. Bidmead duly inserted her into his storyline. The mystery surrounding the Watcher was felt to be a crucial element to hold the viewers' interest, and so it was decided to hold off the Master's first on-screen appearance until Episode Three, in order to make the audience wonder if the Watcher might actually be the evil Time Lord in disguise.

Meanwhile, Nathan-Turner had begun to consider actors for the role of the Fifth Doctor, and found himself uncertain about the direction to take. He was sure about one thing: Baker would cast a long shadow after appearing in Doctor Who for so many years, and thus the new Doctor would have to be as different from Baker's incarnation as possible. Nathan-Turner approached character actor Richard Griffiths -- perhaps best known at the time for his humorous Shakespearean roles and the 1975 film It Shouldn't Happen To A Vet -- but Griffiths was unavailable. Also considered was Scots actor Iain Cuthbertson, who had memorably appeared in programmes such as Budgie, The Stone Tape and Sutherland's Law, and who had played Garron in the 1978 Doctor Who story The Ribos Operation.

Nathan-Turner's office wall was covered with photographs from the various productions on which he had worked. Prior to becoming Doctor Who's producer, one of his most significant assignments had been as production unit manager on the veterinary drama All Creatures Great And Small, from which he had already recruited a number of writers and directors to Doctor Who. Now his eye was caught by one of these photos: a charity cricket match in which All Creatures Great And Small star Peter Davison had been involved. Nathan-Turner realised that the young and fair-haired Davison was a striking visual contrast to Baker; he also knew that All Creatures Great And Small had earned Davison a sizeable fanbase which might follow him to other shows. During October, Nathan-Turner contacted Davison and offered him the role of the Fifth Doctor.

Davison's initial reaction was to turn down the part. He was wary of getting involved in a show like Doctor Who which had virtually become a British institution. He also feared that, at twenty-nine years of age, he was simply too young to play the Doctor. Nathan-Turner was not so easily deterred, however, and gave Davison more time to consider the offer. After a couple of weeks, Davison realised that he simply could not bear to see someone else win the role, and informed the producer of his change of heart.

Janet Fielding was recommended to John Nathan-Turner as a perfect fit to play “a bossy Australian”

At the same time, Nathan-Turner had to find an actress to play Tegan. He was eager to cast an Australian, and more than a hundred performers were interviewed during the autumn. One of the last to audition was Janet Fielding; she had been recommended to Nathan-Turner by a colleague who reckoned that she was a perfect fit to play “a bossy Australian”. At her audition, there was some concern that Fielding was too short to play a flight attendant, but the actress convinced Nathan-Turner that the minimum height requirement for stewardesses in Australia was less than in Britain due to the smaller stature of people from that part of the world. This was Fielding's second fib -- she had also claimed to be three years younger than her true age -- but it paid off. On October 23rd, she was was contracted for three serials beginning with Logopolis, with an option for twelve further episodes. A photocall to introduce the new companion was held the same day.

On October 24th, even more press attention was called to Doctor Who when word leaked out of Baker's imminent departure. Nathan-Turner hastily arranged a press conference, and he and Baker colluded to stir up further publicity with Baker hinting that the Fifth Doctor might be played by a woman -- at the time, an outrageous notion. The truth was revealed on November 4th, when Davison was announced as Baker's successor. On the 11th, the new lead actor was contracted for twenty-eight episodes; at this stage, Season Nineteen was still intended to be the same length as Season Eighteen.

Fielding's first Doctor Who work also marked the beginning of location recording for Logopolis. The director was Peter Grimwade, who had recently completed Full Circle. On December 16th, filming in London began with the material at Aunt Vanessa's residence. When the crew was unable to locate the owner of the house that Grimwade had originally intended to use, they instead moved further up the same Battersea street to the home of Andrew McCulloch, who had co-written Meglos earlier that year. Next, all of the scenes beside the River Thames were completed, with recording taking place on the Albert Bridge and at the Cadogan Pier in Chelsea. Tom Baker rejoined the Doctor Who team just three days after his wedding to former co-star Lalla Ward, who had played Romana.

Grimwade had to proceed without Baker and Matthew Waterhouse on December 17th, when a studio day for The Keeper Of Traken was remounted after being cancelled by an industrial dispute in November. Instead he focussed on material involving Tegan at the Barnet bypass. Plans to use the genuine location were scuppered when it was discovered that the police box which had stood there for many years had recently been dismantled, after falling into disrepair due to persistent acts of vandalism. Fortunately, the TARDIS prop which had been used until the end of Season Seventeen was still in storage. It was refurbished and pressed into use for these sequences, which were recorded at a lay-by in Denham, Buckinghamshire.

On December 18th, the BBC Overseas Monitoring and Receiving Station at Crowsley Park in Sonning Common, Berkshire, posed as the grounds of the Pharos Project. Work at the Denham lay-by then continued on the 19th and 22nd, after which production paused for Christmas. At this point, Bidmead concluded his work as Doctor Who's script editor, leaving interim appointee Antony Root to carry on solo.

One of the first items of business after the holidays was an unusual collaboration between Nathan-Turner and fan adviser Ian Levine. To add to the end-of-an-era feeling of Logopolis, the producer wanted the build to the Fourth Doctor's final moments to incorporate montages of his friends and enemies, each uttering his name. Nathan-Turner was very conscious of Doctor Who's diehard fanbase, and he felt that this sort of flashback -- never before attempted during the series' long run -- would offer enormous appeal. Levine selected the appropriate clips, which were edited together on January 3rd, 1981.

The villains montage included the skeletal Master (from The Deadly Assassin), a Dalek (Destiny Of The Daleks), the Captain (The Pirate Planet), the Cyberleader (Revenge Of The Cybermen), Sontaran Commander Stor (The Invasion Of Time), Broton of the Zygons (Terror Of The Zygons) and the Black Guardian (The Armageddon Factor). The companions montage featured Sarah Jane Smith (from Terror Of The Zygons), Harry Sullivan (The Sontaran Experiment), Brigadier Lethbridge-Stewart (Invasion Of The Dinosaurs), Leela (The Robots Of Death), K·9 (The Armageddon Factor), the original incarnation of Romana (The Stones Of Blood) and the second Romana (Full Circle).

Recording for Logopolis resumed with a two-day studio block on January 8th and 9th at BBC Television Centre Studio 3 in White City, London. Its focus was on material in the TARDIS, with Grimwade capturing action in the console room throughout, in the cloisters on the first day, and in the corridors on the second day. January 9th also saw scenes taped in the antenna control room and atop the gantry itself. Baker had an angry disagreement with Nathan-Turner and Grimwade, who wanted him to yell as the Doctor fell from the scaffolding; Baker felt that this would be unheroic, and refused. The regeneration was also taped on the 9th, and represented Davison's first work on Doctor Who. Baker objected to the fact that the final image that viewers would see of his Doctor would be of him lying prone, being photographed from above.

The Pharos technician attacked by the Master was Robin Squire, who had been Doctor Who's assistant script editor in 1969

The season's last studio session was a three-day affair spanning January 22nd to 24th, and took place in TC6. The first day dealt with sequences in and around the Pharos Project computer room, as well as those in the streets of Logopolis and at the landing area. The Pharos technician attacked by the Master was played by Robin Squire, who had been Doctor Who's assistant script editor in 1969. The second day concentrated on scenes in the External Register and the Control Register. On the last day, all of the material depicting the fall of Logopolis was completed, alongside some modelwork.

Logopolis was the last Doctor Who story to credit Barry Letts as the programme's executive producer, although he would do some work on Four To Doomsday, the first serial of the show's nineteenth recording block. It was now felt that Nathan-Turner had enough experience to guide Doctor Who without additional supervision, and so the position of executive producer was abandoned.

Audiences watched Tom Baker's epic tenure as the Doctor come to a close on March 21st, with the transmission of Logopolis Episode Four. In a deviation from the regular scheduling pattern, this installment was not followed by The Dukes Of Hazzard; instead, a Pink Panther cartoon and Jim'll Fix It joined the usual news update after Doctor Who. This broadcast also marked the end of Doctor Who's seventeen-year run on Saturday evenings; The Dukes Of Hazzard would take over its timeslot the following week, while Doctor Who's return was delayed until 1982 -- when, for the first time, it would air mid-week.

• Doctor Who Magazine #257, 22nd October 1997, “Archive: Logopolis” by Andrew Pixley, Marvel Comics UK Ltd.
• Doctor Who Magazine Special Edition #9, 22nd December 2004, “Another One Bites The Dust” by Andrew Pixley, Panini Publishing Ltd.
• Doctor Who: The Complete History #33, 2017, “Story 115: Logopolis”, edited by Mark Wright, Hachette Partworks Ltd.
• Doctor Who: The Eighties by David J Howe, Mark Stammers and Stephen James Walker (1996), Virgin Publishing.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• In·Vision #52, August 1994, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.