Season eighteen was the first to be produced by John Nathan-Turner, who would remain in charge of the series until 1989. He had spent the previous three years as its production unit manager, and Graham Williams had suggested on a number of occasions that he would be a good candidate to fill a new associate producer post, taking even more of the workload off his shoulders. These suggestions had not been taken up, however, and Williams had ultimately decided to move on.
Nathan-Turner was not the first choice of Head of Serials Graeme McDonald to take over from Williams, but when he was eventually offered the job he gratefully accepted, formally taking over the reins around the beginning of November 1979. McDonald himself was in the process of changing jobs during the latter part of 1979. The Serials Department had been merged with the Series Department in the first major reorganisation of the BBC's Drama Group to have taken place since 1963, and he had been appointed to head the new, larger Department thus created.
Realising that his increased responsibilities left him less time than in the past to devote to individual series, and mindful also of Nathan-Turner's inexperience in his new post, he decided to appoint former Doctor Who producer Barry Letts as executive producer. This in essence simply formalised and expanded an arrangement that had been operating informally since part-way through production of season sixteen, when - while first Nathan-Turner and then David Maloney deputised for Williams, who was absent from the office due to illness - Letts had been given a 'watching brief' over the series. Letts's executive producer role, which would not officially begin until around the second week of June 1980, basically entailed him offering comments on scripts, giving advice and approving major production decisions - in short, taking over the supervisory function normally performed by the head of department.
A new script editor was also required for the series to replace Douglas Adams. First choice was writer and poet Johnny Byrne, and also considered was writer Ted Rhodes. In the end however the post went to Christopher H Bidmead, who had been recommended to Nathan-Turner by former Doctor Who writer Robert Banks Stewart, who at this time was working as producer of the detective series Shoestring.
Although initially reluctant to join the production team, feeling that Doctor Who had abandoned its factual science roots and become 'very silly', Bidmead was won over on discovering that Nathan-Turner and Letts shared his concerns and wanted to return to the more serious style of earlier eras. Bidmead began working on the series in December 1979. With all members of the new production team in place, preparations began in earnest for the forthcoming season - which, as Nathan-Turner had managed to win an allocation of an extra two episodes above the usual twenty-six, would consist of seven four-part stories.
During the early stages of production of season eighteen John Nathan-Turner had reached a mutual agreement with Tom Baker that the actor would bow out of the series upon the expiry of his current contract, bringing to an end his unprecedented seven year run as the Doctor. The producer had not been particularly happy with Baker's portrayal of the Doctor, considering that his increasingly assured and flippant interpretation made the character seem too dominant and invulnerable. He had also greatly disliked the general air of jokiness that Baker tended to inject into the proceedings. Baker, too, had come to feel that now was the right time for him to be moving on. So it was that on 24 October 1980, the day before transmission of the first episode of Full Circle, the BBC held a press conference to announce that the actor would be leaving at the end of the season.
This left Nathan-Turner not only with the task of finding a new leading man but also with the problem of trying to retain the loyalty of the series' regular audience. Baker had won an enormous following, and many were no doubt unaware that there had ever been other Doctors. To recast the lead role in a long-running and popular show is always a high-risk endeavour, and Nathan-Turner felt that Baker's successor would be bound to face an uphill struggle to win acceptance.
One strategy that the producer developed with a view to alleviating this problem was to make the last two stories of season eighteen and the first of season nineteen a loosely-linked trilogy reintroducing the Master. Anthony Ainley was chosen as the new incarnation of the Doctor's arch-enemy largely on the strength of his portrayal of the villainous Reverend Emilius in The Pallisers - a 1974 BBC classic serial on which Nathan-Turner had worked as production manager.
Another idea that Nathan-Turner had considered with the aim of enticing viewers to stick with Doctor Who was to replace Romana with an already established companion character from the series' past. To this end he had approached both Elisabeth Sladen, who had played Sarah Jane Smith, and Louise Jameson, who had played Leela, to see if they would be interested in reprising their roles. Both had declined, so he had then decided instead that two completely new companions should be introduced toward the end of season eighteen - the theory being that these would quickly gain their own respective admirers who would want to see how they coped with the new Doctor.
The first of the two to appear on screen was Nyssa. She had originally been created by writer Johnny Byrne as a one-off character for The Keeper of Traken, but Nathan-Turner had quickly decided to keep her on for at least a further three stories. The second new regular, Tegan Jovanka, made her debut in Logopolis. Created by Nathan-Turner and Bidmead before it was realised that Nyssa would also be a companion, she was originally to have had a three story 'trial run' but later became thought of as a more long-term regular.
Her initial character description established her as a bossy, argumentative air hostess from Australia - a country of origin chosen by the producer partly in order to break the precedent of exclusively British human companions and partly, it has been suggested, with a view to obtaining co-production money from the series' Australian broadcasters, ABC. The actress chosen for the role was Janet Fielding, whose name had been put forward to Nathan-Turner by the Actors Alliance organisation, of which she was a member, on the basis that she was a genuine bossy Australian!
Tom Baker's era as the Doctor had been not only the longest but also arguably the most successful in Doctor Who's history. When the series returned, it would have a new leading man in the person of twenty-nine year old Peter Davison.