Serial 4T:
The Invisible Enemy


A parasite infects the Doctor whilst the TARDIS is hovering in space, and begins to slowly take over his mind. While the Doctor places himself in a coma to stall the organism, 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, Leela has miniaturised clones of herself and the Doctor created, so that they can travel into the Doctor's brain and take the battle to the parasite itself.


The fifteenth season of Doctor Who began under a cloud. Graham Williams had just taken over as producer from Philip Hinchcliffe, and was immediately confronted with a directive from Head of Serials Bill Slater to tone down the programme's levels of violence and horror. These aspects had fostered controversy over the past several years, with Season Fourteen's The Deadly Assassin inviting particular criticism. Slater wanted Williams to take Doctor Who in a more family-oriented direction; to this end, both Williams and Tom Baker decided that humour should feature more prominently than it had under Hinchcliffe.

Williams was also quickly faced with a major upheaval in his own vision for Doctor Who. The new producer disliked the random nature of the Doctor's adventures, and wanted to introduce some structure to the series by having an entire season revolve around a recurring storyline. Furthermore, Williams sought to portray the Doctor as a less anarchic, more responsible figure. At first, Williams mooted a return to the UNIT format which had been prevalent during Jon Pertwee's time as the Third Doctor, but Slater vetoed this suggestion. By the end of November 1976, Williams had instead developed the notion of the Doctor being despatched on a quest through time and space for the six components of an all-powerful “Key To Time”, acting on behalf of a nigh-omnipotent figure called the White Guardian. Unfortunately, Williams quickly discovered that there was no time to implement this story arc for Season Fifteen, and decided to defer the idea until the following year.

It was too late to implement Graham Williams' season-long story arc involving the quest for the Key To Time

Instead, he and script editor Robert Holmes began to seek out more traditional Doctor Who fare. On January 14th, 1977, four scripts titled “Invisible Invader” were commissioned from Doctor Who stalwarts Bob Baker and Dave Martin, whose most recent contribution to the series had been the previous year's The Hand Of Fear. Their starting point was a newspaper article about virus mutations, which suggested to the Bristol Boys that an intelligent, adaptive virus could make an effective antagonist. They married this notion to ideas inspired by the 1966 feature film Fantastic Voyage, in which scientists are reduced to microscopic size and injected into the bloodstream of a dying diplomat.

One notable feature of Baker and Martin's scripts was their decision to use “Finglish” -- phonetic English -- throughout. For instance, words like “Entrance” and “Exit” would be spelt “Entruns” and “Egsit”, respectively. On The Hand Of Fear, the Bristol Boys had found the catchphrase “Eldrad must live” to be a popular ingredient, and so they developed a similar mantra for “Invisible Invader” in the form of the oft-repeated “Contact has been made”. At one stage, once the Nucleus of the Swarm had entered the macroscopic world, its baleful influence would have caused all of the infectees to transform into similar monsters.

The most striking element of “Invisible Invader” was a talking robot dog. This character was originally called FIDO which, in the phonetic vernacular of the script, stood for “Phenomenological Indication Data Observation unit”. Martin was a dog lover whose own pet had recently been run over; FIDO was developed from the idea that if only his dog had been “built like a tank”, the animal would have survived the accident. Subsequently, the name Pluto was considered (but quickly rejected, for fear of confusion with the Walt Disney cartoon character of the same name), after which the character was christened K·9.

Both Williams and Holmes were fond of K·9. Williams in particular thought that the metal mutt would appeal tremendously to Doctor Who's younger viewers. Thought was given to retaining K·9 beyond “Invisible Invader”, and so considerable effort was spent on developing the correct look for the robot dog, with two visual effects designers -- Ian Scoones and Tony Harding -- submitting ideas. Scoones' approach was for K·9 to be a large, armoured machine who could be operated by a small actor. Williams discarded this notion because he felt that K·9 should not be seen as simply a man in a costume. Harding's initial attempt was very cartoonish; he was encouraged to make this more streamlined and functional, while retaining the central concept that K·9 would be radio-controlled. Despite all of this work, however, Williams was cautious about the feasibility of the character, and decided to defer a final decision on K·9's future until he was able to judge how well the prop worked in practise.

Graham Williams rejected one design which envisioned K·9 as a large, armoured machine operated by a small actor

Although “Invisible Invader” was intended to be the second story broadcast as part of Season Fifteen, following Terrance Dicks' “The Vampire Mutation”, it was planned to enter production first. When Baker and Martin's scripts began to fall behind schedule, however, the two serials briefly exchanged filming dates. During February, Graeme McDonald (who had recently replaced Slater as Head of Serials) became concerned that “The Vampire Mutation” might undermine a serious version of Bram Stoker's Dracula in preparation at the BBC for a Christmas 1977 broadcast. Consequently, Dicks was forced to go back to the drawing board and began working on a new adventure, Horror Of Fang Rock. However, there was no way that this would be completed in time to make the required production dates.

As a result, the story now called “The Invisible Invader” was once again brought forward to the pole position of Doctor Who's fifteenth recording block. Designated Serial 4T, it would be directed by Derrick Goodwin. Goodwin's career in television had largely centred on comedy shows (such as On The Buses and Thick As Thieves), although he had also worked on an episode of Z Cars. Goodwin later had to decline an invitation from Williams to return to Doctor Who due to scheduling conflicts, so this turned out to be his only work on the programme. As production loomed, other titles for Serial 4T were considered, including “The Invader Within” and “The Enemy Within”, before The Invisible Enemy was selected.

With K·9 now a candidate for an ongoing place in Doctor Who, the selection of an actor to voice the robot dog took on an increased importance. Goodwin had recently renewed the acquaintance of John Leeson, an actor he had first met while working in repertory theatre. On television, Leeson had appeared in comedy and children's programmes, including Dad's Army and Rainbow, and had also been a BBC continuity announcer. On March 17th, he was contracted to voice K·9 for the final three episodes of The Invisible Enemy. Eight days later, his duties were extended to include the Voice of the Nucleus as well. Also amongst Goodwin's cast was his wife, Elizabeth Norman, playing Professor Marius' nurse.

Production of The Invisible Enemy began on March 22nd, when an experimental effects session took place in Studio 6 of BBC Television Centre. The serial's lavish modelwork was then filmed at Bray Studios from March 28th to April 1st. This was to have been preceded by a week of live-action filming at the Ealing Television Film Studios, but this was cancelled in favour of an extra studio day. As such, The Invisible Enemy was made in two three-day studio blocks, all in TC6.

John Leeson took K·9's place during rehearsals, even getting down on hands and knees beside his fellow performers

By now, Harding had finalised K·9's design, but the construction of the prop's remote control mechanisms had to be contracted out to a firm called the Radio Control Model Centre. Because the Doctor Who budget did not allow for operator Nigel Brackley to be hired before the actual recording dates, Leeson himself decided to take K·9's place during rehearsals, even getting down on hands and knees beside his fellow performers. One element of the K·9 prop that was very nearly overlooked was the ticker tape which was emitted from the robot dog's mouth: this omission was discovered at a very late stage, necessitating the hasty enlargement of K·9's head to fit the device.

The first studio block took place from April 10th to 12th. The 10th and 11th were devoted to scenes in the shuttle, the refuel station, the supervisor's office, and the mess room. April 12th, meanwhile, concentrated on all of the material set within the Doctor's body. Two weeks later, the second studio session spanned April 24th to 26th. The first day included the debut of the new TARDIS console room. With the wooden version introduced the previous year having warped in storage, Williams opted to return to a simplified variation on the predominantly white, more futuristic set employed during Doctor Who's first thirteen seasons. This was conceived by longtime Doctor Who designer Barry Newbery, who elected to simply retain the TARDIS console prop which had been used prior to Season Fourteen.

Tom Baker grew frustrated with the K·9 prop, occasionally giving it a solid kick

The remainder of the second studio block involved sequences in the Bi-Al Centre reception area, the isolation ward, the eye section, and various corridors. The production found itself under increased pressure compared to recent seasons, because one of Williams' instructions from the BBC was to reign in Doctor Who's sometimes runaway budgets, often resulting from lengthy shooting overruns. Unfortunately, the K·9 prop quickly proved itself to be a source of frustration in this regard when it was discovered that the remote control mechanism sometimes interfered with the cameras, resulting in visual distortion on the recordings and the prop itself going haywire. Consequently, Tom Baker quickly grew frustrated with K·9, occasionally going so far as to give the prop a solid kick. Baker also disliked the fact that the robot dog's short stature meant that he often had to stoop down so that they could be in the same shot together. Fortunately, however, Baker and Leeson quickly developed a strong camaraderie.

The script for The Invisible Enemy had been structured so that the scene in which K·9 departs with the Doctor and Leela could simply be dropped if the character was not to be retained, leaving the implication that he had simply been returned to Professor Marius off-screen. Soon after production on Serial 4T concluded, however, Williams decided to keep K·9 on Doctor Who for the remainder of Season Fifteen, despite the problems that the prop had posed. K·9 was revealed to the press on June 10th. By this time, Tony Harding was already formulating plans to overhaul the robot dog's inner workings, in the hope of one day ensuring that K·9 would be no more disuptive a presence in the studio than any other actor.

  • Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing, ISBN 0 426 20369 8.
  • Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing, ISBN 1 85227 444 1.
  • Doctor Who Magazine #271, 18th November 1998, “Archive: The Invisible Enemy” by Andrew Pixley, Marvel Comics UK Ltd.
  • Doctor Who Magazine Special Edition #8, 1st September 2004, “Nobody Does It Better” by Andrew Pixley, Panini Publishing Ltd.
  • In·Vision #25, July 1990, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.

Original Transmission
Episode 1
Date 1st Oct 1977
Time 6.20pm
Duration 23'09"
Viewers (more) 8.6m (40th)
· BBC1 8.6m
Episode 2
Date 8th Oct 1977
Time 6.04pm
Duration 25'13"
Viewers (more) 7.3m (55th)
· BBC1 7.3m
Episode 3
Date 15th Oct 1977
Time 6.13pm
Duration 23'28"
Viewers (more) 7.5m (65th)
· BBC1 7.5m
Episode 4
Date 22nd Oct 1977
Time 6.13pm
Duration 21'22"
Viewers (more) 8.3m (50th)
· BBC1 8.3m
Appreciation 60%

Doctor Who
Tom Baker
Louise Jameson
Michael Sheard
Brian Grellis
Edmund Pegge
Jay Neill
Anthony Rowlands
Nucleus Voice
John Leeson
Professor Marius
Frederick Jaeger
Roy Herrick
Marius' Nurse
Elizabeth Norman
Reception Nurse
Nell Curran
K·9 Voice
John Leeson
Jim McManus
Roderick Smith
Kenneth Waller
A Medic
Pat Gorman
John Scott Martin

Written by
Bob Baker and
Dave Martin
Directed by
Derrick Goodwin
Produced by
Graham Williams

Incidental Music by
Dudley Simpson
Special Sound
Dick Mills
Production Assistant
Norman Stewart
Production Unit Manager
John Nathan-Turner
Brian Clemett
Michael McCarthy
Visual Effects Designers
Ian Scoones
Tony Harding
Film Cameraman
Nick Allder
Costume Designer
Raymond Hughes
Make-up Artist
Maureen Winslade
Script Editor
Robert Holmes
Barry Newbery

Working Titles
[The] Invisible Invader
The Invader Within
The Enemy Within

Updated 8th June 2008