En route to a base on Titan in the year 5000, the crew of a spaceship is infected with an intelligent, malignant virus. A distress signal from Titan is picked up by the TARDIS, but the Doctor also becomes infected. With his mind slowly succumbing the viral Nucleus, the Doctor places himself in a coma. Leela helps him reach a medical facility called the Bi-Al Foundation, where they meet Professor Marius and his robot dog, K·9. Marius creates clones of the Doctor and Leela, whom he miniaturises and injects into the Time Lord's body. They plan to travel into the Doctor's brain and take the battle to the Nucleus itself.

Work on Doctor Who's fifteenth season began under a cloud. At the start of November 1976, Graham Williams began trailing Philip Hinchcliffe in preparation for taking over as producer. He immediately found himself confronted with a directive from Head of Serials Bill Slater to scale back the levels of violence and horror depicted in the programme. These elements had fostered considerable controversy in recent years, and Slater wanted Williams to take Doctor Who in a more family-oriented direction. To this end, Williams and his leading man, Tom Baker, agreed that humour should feature more prominently in Doctor Who than had been the case under Hinchcliffe.

Williams was also quickly faced with the practical implications of his plans for Doctor Who. Disliking the random nature of the Doctor's adventures, he wanted to introduce some structure to the series by having the new season revolve around a recurring storyline. Williams also sought to portray the Doctor as a less anarchic, more responsible figure. At first, he mooted a revival of the United Nations Intelligence Taskforce (UNIT), which had been prevalent throughout the early Seventies, but Slater vetoed this idea. By the end of November, 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. However, script editor Robert Holmes had already begun lining up scripts for Season Fifteen, not all of which would accommodate Williams' proposed story arc. The producer decided to defer his plans to the following year.

A newspaper article about virus mutations inspired the idea of an intelligent, adaptive virus

Amongst the serials Holmes was developing was a story idea from Doctor Who veterans Bob Baker and Dave Martin, whose most recent contribution to the series had been the previous season's The Hand Of Fear. Their starting point was a newspaper article about virus mutations, which inspired the idea that an intelligent, adaptive virus could make an effective antagonist. An item in Scientific American suggested the notion of diseases impacting the mind and imagination. Baker and Martin also drew upon the 1966 feature film Fantastic Voyage, in which a team of specialists was reduced to microscopic size and injected into the bloodstream of a dying scientist. The resulting scripts, entitled “Invisible Invader”, were commissioned on January 14th, 1977.

Baker and Martin originally hoped that much of Episode Three could be made on location, with the Doctor's mind represented by college cloisters or a stately garden. They also intended the Nucleus' emergence into the macroscopic world to trigger the transformation of its infected victims into similar creatures. At one stage, the adventure's resolution involved the Doctor igniting Titan's atmosphere in order to destroy the Nucleus and its breeding tank. When writing The Hand Of Fear, Baker and Martin had found the introduction of the catchphrase “Eldrad must live” to be effective. As such, they developed a similar mantra for “Invisible Invader” in the form of the oft-repeated “Contact has been made”. To emphasise the far-future setting, they decided to use “Finglish” -- phonetic English -- throughout. For instance, they indicated that words which would appear on signs, like “Entrance” and “Exit”, should be spelt “Entruns” and “Egsit”.

The most striking element of “Invisible Invader” was a talking robot dog, introduced as a way of providing information to the viewer while the real Doctor was unconscious and the clone Doctor was inside his body. Martin was a dog lover who had recently suffered the loss of two pets beneath the wheels of vehicles. The new character was inspired by the notion that, had his dogs been “built like a tank”, they would have survived the accidents. Originally, the writers conceived the name FIDO which, in the phonetic vernacular of the script, stood for “Phenomenological Indication Data Observation unit”. Pluto was subsequently considered -- but quickly rejected, for fear of confusion with the Walt Disney cartoon character of the same name -- after which the robot dog was christened K·9.

Both Williams and Holmes were fond of K·9. Williams, in particular, thought that the robot dog offered tremendous appeal to Doctor Who's younger viewers, and began to consider retaining him beyond “Invisible Invader”. As such, considerable effort was spent on developing the correct look for K·9, 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, on the other hand, thought that the robot dog should be a radio-controlled prop. His initial concept was very cartoonish, and he was encouraged to make it more streamlined and functional. 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 practice.

Although Invisible Invader was to be the second story of Season 15, it would enter production first

Although “Invisible Invader” was intended to be the second story broadcast as part of Season Fifteen, following Terrance Dicks' “The Vampire Mutation”, it was scheduled to enter production first. Some sources have suggested that the recording order was later changed to conform with the transmission plans, and then swapped back as a result of the replacement of “The Vampire Mutation” with Dicks' Horror Of Fang Rock. However, it is more likely that the original plan remained in place throughout.

As recording approached, the title of Baker and Martin's serial was slightly amended to “The Invisible Invader”. However, other options were being considered, including “The Invader Within” and “The Enemy Within”; The Invisible Enemy was ultimately selected. Its director would be Derrick Goodwin, making his only Doctor Who serial. He had recently renewed the acquaintance of a former colleague called John Leeson, who was an experienced vocal performer and was looking for work. On March 17th, Leeson was contracted to provide K·9's dialogue 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 partner, Nell Curran, who played the Reception Nurse; they would later marry.

Eager that his first Doctor Who serial be visually impressive, Williams allocated unusually lavish model filming to The Invisible Enemy. This work took place at Bray Studios in Water Oakley, Berkshire from March 28th to April 1st. It was to have been preceded by a week of live-action filming at the BBC Television Film Studios in Ealing, London. However, this was cancelled in favour of an extra studio day, which meant that The Invisible Enemy would be made in two three-day recording blocks. The venue in both cases was BBC Television Centre Studio 6 in White City, London.

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 Nigel Brackley of the Radio Control Model Centre in Harlington, London. One element of the K·9 prop that was very nearly overlooked was the ticker tape which was to be emitted from his mouth. This omission was discovered at a very late stage, necessitating the hasty enlargement of K·9's head to fit the mechanism. Because the Doctor Who budget did not allow for Brackley to be hired to operate K·9 before the actual recording dates, Leeson himself decided to take the robot dog's place during rehearsals, even getting down on his hands and knees beside his fellow performers. Meanwhile, Leeson was also contemplating his delivery of K·9's dialogue. Because the prop was fitted with a tartan collar, a Scottish accent was briefly considered.

Fibreglass tended to flake off the Nucleus costume, becoming a breathing hazard

The first studio block, originally scheduled for April 10th and 11th, was now extended to include the 12th as well. The first two days were principally devoted to material in the Titan Base refuel station, supervisor's office and corridor. Scenes in the shuttle were also taped on the 10th, and those in the mess room on the 11th. Next, April 12th concentrated on all of the material set within the Doctor's body. Because of the money spent on K·9 and the model sequences, little was left over to construct the Nucleus costume. The result, inspired by scripted references to its prawn-like appearance, was not popular with the cast and crew, and was very uncomfortable for actor John Scott Martin to wear. To make matters worse, bits of fibreglass tended to flake off, sticking to the camera lenses and becoming a breathing hazard.

Another source of frustration was the K·9 prop. It was discovered that its remote control mechanism sometimes interfered with the cameras, resulting in visual distortion and the prop itself going haywire. K·9 also couldn't be rolled too quickly without suffering damage, and struggled to traverse even small irregularities. Tom 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. Consequently, he became easily aggravated by K·9, occasionally giving the prop a solid kick. Fortunately, however, Baker and Leeson quickly developed a strong camaraderie, and this helped ease the strained nature of the relationship between Baker and Louise Jameson.

One of the directives given to Williams when he took the reins of Doctor Who was to exercise more careful control the programme's budget. Several serials during Hinchcliffe's tenure had been characterised by costly overruns on studio days, but it was now made clear that such expenses would no longer be routinely authorised. As a result, directors would have to ensure that all of the required material was recorded during the normal studio hours, even in the face of unexpected problems and delays. These instructions were almost immediately challenged by the amount of material which Goodwin had to record on the 11th, together with the necessity of clearing and ventilating the studio because of the issues with the Nucleus costume. As a result, Williams found himself forced to sanction almost an hour of additional studio time.

The second recording 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 prior to Season Fourteen. The new set was conceived by longtime Doctor Who designer Barry Newbery, who refurbished and incorporated the TARDIS console prop used during Season Thirteen. Apart from the TARDIS scenes, the second block was entirely dedicated to material in the Bi-Al Foundation. Goodwin taped sequences in various corridors on all three days, together with those in the reception area on the 24th, the isolation ward on the 25th, and the ophthalmology section on the 26th.

Soon after production concluded, Graham Williams decided to keep K·9 on Doctor Who

The script for The Invisible Enemy had been structured so that the scene in which K·9 departed with the Doctor and Leela could simply be omitted if the character was not to be retained, leaving the implication that he had been returned to Professor Marius off-screen. Soon after production 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. On June 10th, the British press first revealed K·9's forthcoming addition to the TARDIS crew. This was followed by a photocall on October 6th. Meanwhile, Tony Harding was formulating plans to overhaul the robot dog's inner workings, in the hope of ensuring that K·9's presence in the studio would be no more disruptive than that of any other actor.

The planned timeslots for The Invisible Enemy varied between 6.05 and 6.15pm, influenced partly by the number of cartoon shorts which aired earlier in the evening, and partly by a change of lead-in. Star Turn ended its run just before Episode Two aired on October 8th. The following week, The Basil Brush Show returned to the BBC Saturday evening schedule in its place.

• 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.
• Doctor Who: The Complete History #27, 2017, “Story 93: The Invisible Enemy”, edited by John Ainsworth, Hachette Partworks Ltd.
• Doctor Who: The Handbook: The Fourth Doctor by David J Howe, Mark Stammers and Stephen James Walker (1992), Virgin Publishing.
• Doctor Who: The Seventies by David J Howe, Mark Stammers and Stephen James Walker (1994), Virgin Publishing.
• In·Vision #25, July 1990, “Production” edited by Justin Richards and Peter Anghelides, Cybermark Services.
• P.S.: The Myth Of Fang Rock by Paul Scoones (2024).