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Episode Guide
The Invisible Enemy
Production Code: 4T
Season 15, Story Number 93
Directed by Derrick Goodwin
No episode stills are currently available for this story.

Each episode is identified with date of transmission, duration, ratings in millions, and (for 1963-1974 only) archive status.

Part One
01 October 1977 | 23'09" | 8.6
Part Two
08 October 1977 | 25'13" | 7.3
Part Three
15 October 1977 | 23'28" | 7.5
Part Four
22 October 1977 | 21'22" | 8.3
Archive Status: All four episodes exist as PAL 2� colour videotape, always held by the BBC�s Film and Videotape Library.
Tom Baker (Doctor Who), Louise Jameson (Leela), John Leeson (K9), Frederick Jaeger (Professor Marius), Michael Sheard (Lowe), Brian Grellis (Safran), Edmund Pegge (Meeker), Jay Neill (Silvey), Anthony Rowlands (Crewman), John Leeson (Nucleus Voice), Roy Herrick (Parsons), Elizabeth Norman (Marius' Nurse), Nell Curran (Reception Nurse), Jim McManus (Opthalmologist), Roderick Smith (Cruikshank), Kenneth Waller (Hedges), Pat Gorman (A Medic), John Scott-Martin (Nucleus)
Hovering in space around the year 5000 AD, the TARDIS enters a cloud of energy which shoots lightning-like tendrils into its systems and from there into the Doctor's mind. He has been infected by the Virus, a space-borne intelligence which wishes to spread itself across the universe.

The TARDIS arrives on Titan, one of the moons of Saturn, where a refuelling station has also been taken over by the Virus. The Doctor manages to relay the coordinates of a local hospital asteroid to Leela before he collapses.

At the Bi-Al Foundation, based on asteroid K4067, Professor Marius examines the Doctor and pronounces that there is little he can do. However the Doctor devises a plan whereby he clones himself and Leela, miniaturises the clones using the relative dimensional stabiliser from the TARDIS, and then has them injected by Marius into his body, where they have to find and destroy the Virus 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 prior to its invasion of the galaxy.

The Doctor, now cured of the Virus's influence, arranges with the help of K-9, Professor Marius's dog-shaped robot computer, for the breeding tanks to be blown up, thus killing the Nucleus.

Marius gives K-9 to the Doctor as a parting gift as the Professor is returning to Earth.

Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted.

Production Team
Tony Garrick (Assistant Floor Manager), Christabel Albery (Assistant Floor Manager), Raymond Hughes (Costumes), Barry Newbury (Designer), Nick Allder (Film Cameraman), Glenn Hyde (Film Editor), Dudley Simpson (Incidental Music), Maureen Winslade (Make-Up), Graham Williams (Producer), Norman Stewart (Production Assistant), John Nathan-Turner (Production Unit Manager), Robert Holmes (Script Editor), Dick Mills (Special Sounds), Brian Clemett (Studio Lighting), Michael McCarthy (Studio Sound), Delia Derbyshire (Theme Arrangement), Ron Grainer (Title Music), Tony Harding (Visual Effects), Ian Scoones (Visual Effects)
Story Notes
The lackluster "The Invisible Enemy" is remembered to this day for one important addition to the series: K-9, the robot dog designed by Tony Harding and built by the BBC Visual Effects department, who would become one of the series' most popular trappings. John Leeson, who voiced the Nucleus in this story, joined the cast as the voice of K-9, which he would do for three of the four forthcoming seasons; the idea to keep the character on was made very late in production so that some of the following stories had to be rewritten to incapacitate the dog. This story saw a return to the original-style TARDIS control room, abandoned with the use of the secondary room in "Masque of Mandragora," although the room was redesigned by Barry Newbury. Graham Williams became producer with this story, which was filmed first in the season. Originally this story would have featured the Key to Time, but the stories could not be reworked at a late date so the storyline was postponed one year. The working titles for the story were "The Invader Within," "The Enemy Within" and "The Invisible Invader." K-9 was originally to be called Pluto, changed to avoid the wrath of the Walt Disney Corporation.
For more in-depth information about the contents of this story, a complete episode-by-episode detailed breakdown can be found at the Doctor Who Reference Guide.
Additional, more detailed information about the production of this story can be found at Shannon Patrick Sullivan's A Brief History of Time (Travel).
Video release
Released as �The Invisible Enemy� in the UK [September 2002] and Australia/New Zealand [November 2002] (BBC catalog #7267), US/Canada [October 2003] (WHV catalog #E1859); episodic format, photomontage cover. US/Canada version sold individually as well as part of the "End of the Universe Collection" 13-tape set [October 2003] (WHV catalog #E1840). The cover illustration for the final release is modified from the original design circulated electronically throughout fandom (specifically, Leela's pose and the arch background).
In Print
Novelised as �Doctor Who and the Invisible Enemy� by Terrance Dicks (Target #36), first released in 1979 with cover art by Roy Knipe. Released in hardcover in 1979. Originally planned to be re-released in 1994 with cover art by Alister Pearson; the release was canceled while Pearson's artwork remained unfinished.
For more details on the various novelizations of this story, with additional background material, artwork and details of both UK and foreign releases, visit On Target.