An early priority for Graham Williams and Douglas Adams in their preparations for Doctor Who's seventeenth season was to decide how to replace Mary Tamm, who had resisted all attempts to persuade her to stay on for a further year as Romana. They concluded that the simplest plot explanation for Tamm's sudden disappearance would be to have Romana regenerate. Although they toyed with the idea of casting a different actress in the role for each story, they quickly realised that this would be impractical.
Eventually they offered the job to Lalla Ward, who had impressed them with her performance as Princess Astra in The Armageddon Factor and, importantly, had got on well with Tom Baker. John Leeson had also decided to move on at this point, and so someone new was required to voice K9. David Brierley was cast after two rounds of auditions by director Christopher Barry, who had been assigned to handle his debut story The Creature from the Pit. The season's first transmitted story, meanwhile, would see the robot dog out of action with 'laryngitis' as the Doctor faced once again his most notorious adversaries...
Season seventeen proved to be a highly popular one with viewers, its ratings showing a marked reversal of a gradual downward trend seen during seasons fifteen and sixteen. Indeed City of Death received the highest ratings ever recorded for a Doctor Who story, the final episode reaching 16.1 million viewers - although this exceptional feat was admittedly due to the fact that a strike had blacked out the ITV network at the time, which meant that the series' only competition came in the form of minority interest programmes on BBC2.
Behind the scenes, however, more changes were in store for the series. Douglas Adams had decided to relinquish the script editor's post after just one season, mainly because he was becoming increasingly busy with The Hitch-Hiker's Guide to the Galaxy. Even more significantly, Graham Williams had decided during production of Nightmare of Eden that he too would leave the series on completion of this season. His three year period as producer had arguably been the most problem-hit of any in the series' history, and he now felt in need of a break. He and Adams therefore left together, opening the way for a new production team to steer the series onto a different course for the eighties.