Starring: Tom Baker (The Fourth Doctor), Louise Jameson (Leela), John Leeson (Voice of K-9).
The Doctor, Leela and K-9 find themselves in a spacecraft piloted by the last of the Minyans, a race which destroyed itself using technology given to them by early Time Lords. Now the remaining Minyans are on a desperate search for their race banks, lost centuries earlier, which represent the only hope for the survival of their civilisation. With the Doctor's help, the race banks are located. But in order to retrieve them, the time travellers and the Minyans must confront an insane computer and its robotic servants, or the Minyan race will be forever doomed.
Anthony Read fully took over from Robert Holmes as Doctor Who's script editor for the last two serials of Season Fifteen, after trailing Holmes for much of the year. It quickly became obvious that Read would have his hands full for the planned finale for the season, The Killer Cats Of Geng Singh (spelling varies) by David Weir. Consequently, for the penultimate serial he turned to two Doctor Who veterans, Bob Baker and Dave Martin. The Bristol Boys had already written The Invisible Enemy earlier the same year.
Although Read and producer Graham Williams were under pressure from their superiors to move away from the level of horror included in the series under Holmes and former producer Philip Hinchcliffe, Read did intend to continue his predecessor's tradition of borrowing liberally from classic literature. He suggested Baker and Martin might look to Greek mythology for a storyline, and in particular the Argosy -- the quest by Jason and other Greek heroes, the Argonauts, to find the fabled Golden Fleece. What resulted was an adventure first entitled Underground and then Underworld which drew numerous parallels with the Greek legend. In particular, most of the names in the serial are derived from mythology, including Jackson (Jason), Herrick (the Argonaut Heracles), Orfe (the Argonaut Orpheus), Tala (Atalanta, who tried unsuccessfully to join Jason's quest), Idas and Idmon (both minor Argonauts), and the P7E (Persephone, the goddess of fertility who was kidnapped and forced to stay in the Underworld). The Oracle was named after the ancient seers, the most famous of which was located at Delphi. The Minyan ship was originally called the R1 (after Jason's ship, the Argo) and then rechristened the R1C (after the Argosy, perhaps a more evident parallel). Several other character names and situations have similar derivations.
The director assigned to Serial 4Y was Norman Stewart. Stewart had just gone freelance after working at the BBC for a number of years, having handled series such as The Newcomers. He had also worked on several Doctor Who stories as a production asistant. A challenge immediately confronted Stewart after Williams attended a preview screening of the motion picture Star Wars which was already a hit in North America and would debut in the UK in early 1978, around the time of Underworld's transmission. Williams was aware that Doctor Who's usual production standards would compare poorly with Star Wars. Consequently, he and Stewart decided to abandon performing any material on the film stage at Ealing, and instead invest the majority of the serial's budget into two impressive sets: the R1C (which could be redressed as the P7E) and the caverns.
At this point, Williams elected to go on holiday for two weeks. When he returned, he discovered the end of Season Fifteen in complete disarray. Doctor Who was under orders not to go overbudget, as had often been the case under Hinchcliffe. However, inflation was skyrocketing in 1977, and insufficient funds remained for both Underworld and the six-part finale. This left Williams with two options: either completely rethink the Underworld production, or lose the last serial. Work had already progressed significantly on the R1C set. But Williams suggested that, instead of constructing the cave set, it might be possible to instead just build a model and insert the cast "into" the miniaturised set using Color Separation Overlay (CSO). Stewart and designer Dick Coles investigated the idea, and finally determined that it was feasible, despite the fact that such a volume of CSO work had never before been attempted in a BBC production.
Taping on Underworld was comprised of six studio days, a two-day session on Monday October 3rd and Tuesday the 4th, and then a four-day session running from Saturday the 15th to Tuesday the 18th. Because of the precise nature of CSO work, tensions ran high, and the serial proved very unpopular with the cast. To make matters worse, the CSO effect did not achieve quite the same level of success as the crew had anticipated. The high volume of special effects work also led electronic effects supervisor AJ "Mitch" Mitchell to request, and be granted, a so-called "gallery-only" day. Taking place on October 21st, this was a studio session in which only the recording gallery was used by the production team, without any cast, while preparatory work on another programme proceeded in the studio proper. This was the first gallery-only day in Doctor Who history, although Williams had originally wanted to allocate one to every serial that season.
In between the two studio sessions, Louise Jameson confirmed to Williams that she would not be returning for Season Sixteen. Although she had indicated this to him some weeks previously, during work on The Sun Makers, the producer had held out hope that he might convince Jameson to remain, as he had done the year before. Work quickly began on conceiving a new companion, with an initial character sketch for Leela's replacement, Romana, having been prepared by October 10th. Jameson's imminent departure was announced to the press on November 7th. In a bid to gain extra exposure for her client, the agent of Imogen Bickford-Smith (Tala in Underworld) began publicly touting her as Leela's replacement, even though Williams and Read had never considered it as an option.
Meanwhile, the Doctor's other new companion, K-9, was finally unveiled to the public on October 6th. K-9's creators, Baker and Martin, briefly considered spinning their latest inventions, the R1C crew, off into their own science-fiction series, in which the Minyans travelled through space becoming involved in other mythology-inspired adventures. Nothing would come of this idea, however.