The Invisible Enemy
The TARDIS is infiltrated by the Swarm - a space-borne intelligence that wishes to spread itself across the universe - and the Doctor is infected by its nucleus. The ship then materialises on Titan, one of the moons of Saturn, where the human occupants of a refuelling station have also been taken over.
The Doctor eventually collapses as a result of his infection, but first manages to relay to Leela the coordinates of a local hospital asteroid. At the Bi-Al Foundation, based on the asteroid, Professor Marius clones the two time travellers, miniaturises the clones using the relative dimensional stabiliser from the TARDIS and then injects them into the Doctor's body in the hope that they can find and destroy the nucleus.
The plan backfires as the nucleus escapes from the Doctor in place of the clones and is enlarged to human size. The creature arranges for itself to be taken back to Titan, where breeding tanks have been prepared for it.
The Doctor, now cured of its influence, enlists the help of K9, Professor Marius's dog-shaped robot computer, and sets a booby-trap that results in the breeding tanks being blown up, killing the nucleus. Marius gives K9 to the Doctor as a parting gift.
K9 and Company: A Girl's Best FriendPlot
The gap between seasons eighteen and nineteen was enlivened for Doctor Who viewers not only by The Five Faces of Doctor Who, a BBC2-transmitted run of repeats that for the first time ever featured stories from Doctors other than the current incumbent, but also by a one-off, fifty-minute special that John Nathan-Turner hoped would launch a whole new spin-off series.
Following the public outcry that had occurred when the news that K9 would be leaving Doctor Who filtered out in early June 1980, Nathan-Turner had put forward to his BBC superiors the suggestion that the robot dog be given its own programme. This idea had been approved and a pilot production set in motion. The initial outline for the story was written by Nathan-Turner himself at the end of April 1981, under the working title One Girl and Her Dog. This sketched out a basic idea for a 'black magic yarn' set in an English country village, involving Sarah Jane Smith, her Aunt Lavinia's ward Brendan and K9 mark III - apparently a gift left for her by the Doctor, but in fact under the control of his arch-enemy the Master. In terms of style, the production was envisaged as being more in the mould of The Avengers than of Doctor Who.
At the beginning of May 1981, Nathan-Turner and new temporary Doctor Who script editor Antony Root produced a format document for the programme, now entitled A Girl's Best Friend, which contained character outlines of Sarah and K9 and a refined synopsis of the story, this time with no mention of K9 being under the Master's control. It was shortly after this, on 12 May, that Nathan-Turner first contacted Elisabeth Sladen to see if she would be willing to appear in the special. Fortunately she readily agreed - something she had previously resisted doing when invited first by Graham Williams and then by Nathan-Turner himself to return as a regular in the parent series. She would be joined by John Leeson, who agreed to reprise his role as K9's voice.
Terence Dudley was approached to script the special. After discussions and the preparation of a detailed scene breakdown he wrote a full draft script that stuck quite closely to the ideas developed by the production team but fleshed them out accordingly. Root's successor, Eric Saward, made a number of amendments to the script during September 1981 - by which time the special had acquired its final title of K9 and Company (Nathan-Turner's superiors having requested that it refer explicitly to K9), with A Girl's Best Friend relegated to a subtitle.
Location shooting - including for the title sequence, which Nathan-Turner decided should be in the same style as those for the US series Hart to Hart and Hawaii Five-0 - was carried out in November 1981 near Cirencester in the Gloucestershire countryside. A two-day studio session subsequently took place at the BBC's Pebble Mill studios in Birmingham.
K9 and Company: A Girl's Best Friend, the first bona fide TV spin off from Doctor Who, was eventually seen by viewers on Monday 28 December 1981 as part of the Christmas season on BBC1. In the story as transmitted, Sarah pays a Christmas visit to her Aunt Lavinia's house in the village of Moreton Harwood. She discovers that Lavinia, a noted scientist, has yet to return from a lecture tour of the USA. She does however meet Brendan - Lavinia's ward - and Commander Bill Pollock - her partner in a small market garden business. Also in the house, in a box sent to her by the Doctor, she finds K9. Brendan is kidnapped by a local coven of witches who want to sacrifice him to the goddess Hecate. Sarah, with K9's assistance, foils their plan and unmasks their leaders - Commander Pollock and local postmistress Lily Gregson.
The special won a quite respectable viewing figure of 8.4 million and would no doubt have done even better had it not been for the fact that the Winter Hill transmitter in the North West region suffered a power blackout at the time. The option of a full series was never pursued, however. The programme had a single repeat screening the following Christmas, on 24 December 1982 on BBC2, where it pulled an audience of 2.1 million.