Serial 4T:
The Invisible Enemy

Working Titles: The Invader W
ithin, The Enemy Within, The Invisible Invader.
Starring: Tom
Baker (The Fourth Doctor), Louise Jameson (Leela), John Leeson (Voice of K-9, Voice of the Nucleus).

A parasite i
nfects the Doctor whilst the TARDIS is hovering in space, and slowly takes over his mind. While the Doctor places himself in a coma to stall the parasite, Leela takes the Time Lord to a medical facility on Titan in the far future. There, with the help of Professor Marius and his robot dog K-9, she has miniaturised clones of herself and the Doctor created, so that they can travel into the Doctor's mind and take the battle to the parasite itself.

Doctor Who's fifteenth sea
son began under a cloud. Graham Williams had just taken over as producer from Philip Hinchcliffe, and was immediately confronted with a number of problems. Louise Jameson had been brought on board in the latter half of Season Fourteen to play Leela, intended as an interim companion. Star Tom Baker loathed the character, however, and Jameson herself had some doubts about remaining on the series. Nonetheless, Williams encouraged the actress to return for the next season, and ultimately she agreed. Also, Head of Drama Bill Slater instructed Williams to tone down the violent content which had garnered such notoriety throughout the Seventies -- and in particular during Hinchcliffe's time on the series. Slater wanted Williams to make Doctor Who a more family-oriented programme. And with Hinchcliffe gone, script editor Robert Holmes was also making his desire to leave the show known, especially given his disapproval of the additional levity now required by the BBC. Like Jameson, Holmes was convinced by Williams to stick around, at least for six months until Williams was more comfortable with his new duties.

Despite the many problems he encountered
in assuming control of Doctor Who, Williams also brought with him a number of ideas. In particular, he wanted to construct a season in which all the stories were linked by a theme. Slater had nixed the idea of bringing back the UNIT format prevalent during Jon Pertwee's tenure as the Doctor, and so on November 30th, 1976, Williams devised a brief document describing a story arc in which the Doctor would search for the six segments of an artifact called the Key to Time. Each serial in the season would center around finding one of the segments. Unfortunately, several storylines were already in development for Season Fifteen when Williams came on board, and the Key to Time concept could not be shoehorned into all of them. Consequently, Williams decided to postpone his theme idea for one year.

The first story into production for the fifteenth r
ecording block was supposed to be The Vampire Mutations, by former script editor Terrance Dicks, commissioned on January 11th, 1977, very close to the start of work on the new season. Unfortunately, not long after, Head of Serials Graeme MacDonald ordered that work on The Vampire Mutations be halted, out of fears that it would undermine a serious version of Bram Stoker's Dracula in preparation at the BBC. Consequently, Dicks went back to the drawing board and conceived a new idea, but it would be impossible for him to complete his scripts in time for the start of recording. As a result, the season's second story would have to be made first.

This story was The In
visible Enemy (which had also possessed such working titles as The Invader Within, The Enemy Within and The Invisible Invader), written by the team of Bob Baker and Dave Martin. The Bristol Boys had contributed The Hand Of Fear a year earlier. For this serial, they drew inspiration mainly from the 1966 movie Fantastic Voyage.

Included in Baker and Martin's script
was a role for a robot dog. The machine was originally called Pluto but, due to fears of reprisals from the Walt Disney Company, this was changed to K-9. Holmes was very fond of K-9, and it was felt that the robot had potential as a series regular, fitting in very well with the new "family-oriented" directive. Because of the large number of special effects shots in The Invisible Enemy the serial was assigned two effects designers -- Ian Scoones (modelwork) and Tony Harding (studio effects). Both Scoones and Harding took a hand at conceptualising K-9. Scoones envisaged him as a large creature into which an actor could fit, but Williams wanted something less fierce-looking that did not look like a costume. Consequently, it was decided to proceed with Harding's version of a small, radio-controlled device.

Unfortunately, the K-9 prop would quic
kly prove to be a source of endless frustration. It was soon discovered that the remote control device interfered with the cameras, resulting in visual distortion and K-9 itself going haywire. Partly because of this, recording quickly fell badly behind schedule. Because Williams was now under instructions not to let costs on Doctor Who overrun as had often happened under Philip Hinchcliffe, overtime to complete shots was a luxury which could no longer be afforded. As a result, important material for episode four had to be pared down and ad-libbed during production. Consequently, Tom Baker quickly grew frustrated with K-9, often resorting to kicking the prop. Baker also disliked the fact that the robot dog's short height meant that he often had to stoop down so that they could both be in the same shot.

Fortunately, Baker's agitation with K-9
was somewhat mitigated by his fast friendship with John Leeson, hired to voice the new character. Leeson had done considerable work in sitcoms and children's programmes, such as Dad's Army and Rainbow, and worked as a continuity announcer for the BBC. He thrust himself eagerly into the role, physically replacing the prop in rehearsals by getting down on all fours. Williams and Holmes had not yet decided whether K-9 would indeed be retained for future serials, however. An extra scene was to be recorded in which K-9 joined the Doctor and Leela, which could be included or removed from episode four as necessary.

The director for The Invisible Enemy was Derrick Goodw
in, with whom Williams had worked on Z Cars. It would be his only Doctor Who job. Like many serials in recent years, studio taping for The Invisible Enemy was done on a three-day/two-day basis, with the first block running from Sunday April 10th to Tuesday the 12th, and the last from Sunday the 24th to Monday the 25th. The story also saw the introduction of the second new TARDIS console room in two seasons. The wooden set designed by Barry Newbery the previous year had warped in storage and was unusable. Furthermore, Williams preferred a more futuristic look, and so Newbery opted for a simplified version of the original control room, including the return of the old console prop.

The decision to have K-9 become a new compa
nion was finalised by June 10th, when the news was unveiled to the press. This was in part to offset the significant costs of the prop, since its use in multiple serials better justified the large expenditure. However, because some of the subsequent serials were already completed or at an advanced stage of development, K-9 would play only a small role in these stories.