The Invisible Enemy is the second serial of the 15th season in the British science fiction television series Doctor Who, which was first broadcast in four weekly parts from 1 October to 22 October 1977. The serial introduced the Doctor's newest companion, the robot dog K-9, voiced by John Leeson.
Mankind is colonizing space at a fantastic rate. Some human space travellers are cruising near the outer planets of the solar system with their ship on autopilot. The ship's computer, and soon the human crew, is possessed by a strange virus. Reaching their destination, Titan Base, they proceed to take over the base as a breeding ground. The station manager, Lowe is able to send out a distress call.
The TARDIS is travelling through the same region, and is infected by the virus. The infection passes to the Doctor, but he is unaffected for the moment. He and Leela hear the distress call and go to investigate. While there, the Doctor is overcome by repeated infections and is chosen, due to his incredible powers as a Time Lord, to be the host of the Nucleus of the Swarm. Leela is unable to be infected.
The Nucleus declares her a reject and orders that she be killed. The Doctor manages to break free of his infection and tells Leela how to get the TARDIS to the nearest medical centre. Accompanying them is Lowe, who has been infected, although the Doctor and Leela don't know this.
At the medical station, the Doctor's doctor, Professor Marius, introduces the group to K-9, a robotic dog he made to replace the real dog he had to leave on Earth. Professor Marius is baffled as to how to treat the Doctor's strange infection. Meanwhile, Lowe has been infecting the staff of the hospital.
Leela and the Doctor decide on a last-ditch strategy. They create clones of themselves, which can only survive for ten minutes due to problems with the technique. The clones will then be shrunk and inserted into the Doctor. There they will destroy the Nucleus and escape through a tear duct. In the meantime, Leela and K-9 fight off the infected staff of the hospital.
After a hazardous voyage through his mind, the Doctor's clone and Leela's clone are separated, and the Doctor's clone reaches the Nucleus. He has no weapons with which to destroy it, and it learns the intended escape route of the Doctor's clone, since the Doctor thought of it. Prof. Marius faithfully retrieves something from the tear duct and expands it to human size. It turns out to be the Nucleus. The Doctor is cured of his infection.
The Nucleus and the infected staff leave for Titan Base so the Nucleus can spawn. The Doctor realizes he is cured since Leela's clone introduced into his blood stream her immunity factor. He replicates it and gives it to Prof. Marius. The Doctor, Leela, and K-9 proceed to Titan Base in the TARDIS.
They just barely manage to fight off the infected humans, but are again without sufficient weaponry to destroy the Nucleus, or its many children, which are about to hatch as "macro-sized" beings, like the newly macro-sized Nucleus. The Doctor manages to jam the door they are behind and rigs a gun to fire into a cloud of oxygen gas he is releasing and escapes. As intended, when the Swarm finally forces open the door, the blaster fires, igniting the oxygen in Titan's methane atmosphere and destroying the Swarm and the base.
When they return to the hospital, they thank Prof. Marius for the use of K-9, who has ably assisted them. Prof. Marius offers K-9 to the Doctor, as he is due to return to Earth, and the Doctor and Leela leave with their new companion in the TARDIS.
A new TARDIS console room debuts in this story, replacing the more Victorian, auxiliary console room that had been used for the previous season. That console room set had been exposed to moisture in the break between seasons and warped to the point that it was no longer usable. The re-designed TARDIS console, which had last appeared in Pyramids of Mars, is once again used from this story up until The King's Demons.
In part 2, Gallifrey is assumed to be in Ireland, much the same as Tegan tells her captors in Arc of Infinity. Gallifrey was also assumed to be in Ireland in The Hand of Fear, in "Human Nature", in the novel Blood Harvest and in the audio drama Dark Eyes.
Working titles for this story included The Enemy Within, The Invader Within and The Invisible Invader. It was not decided until late in the production that K-9 was to be a new companion. The decision to use it in multiple serials was made partly to offset the expense that had gone into making the prop.
The Invisible Enemy was filmed and recorded in April 1977. In one scene there is an obvious crack in a wall before it is fired at by K-9; the crack was originally concealed, but the scene was reshot with little time left to repair the join.
Michael Sheard (Lowe) makes his fourth of six appearances in Doctor Who. Brian Grellis previously played Sheprah in Revenge of the Cybermen and would later appear as the Megaphone Man in Snakedance. Frederick Jaeger (Marius) also played Jano in The Savages in 1966 and Professor Sorenson in Planet of Evil in 1975.
Broadcast and reception:
Serial details by episode
Viewers, (in millions)
1 October 1977 (1977-10-01)
8 October 1977 (1977-10-08)
15 October 1977 (1977-10-15)
22 October 1977 (1977-10-22)
Reviewing the serial for The Times newspaper on the Monday following the second episode's transmission, critic Stanley Reynolds gave the story a generally negative reception. He also pointed out that in ITV regions where the series was competing with Man from Atlantis, it was now losing the ratings war.
"In the current story, The Invisible Enemy, now halfway through its four-week run, a malignant virus has struck a space station," wrote Reynolds. "Some evil force is attempting to take the station, and undoubtedly the universe, over. When one is being 'taken over' those sort of lightning flashes like the advert for learning how to hypnotize, travel from the eyes of the villain to the one having the fluence put upon him. One then gets rather furry of face and hand, but the appeal of Dr Who has always been the monster and this time out the BBC seems to have lost its touch with monsters... Maybe the leggy Leela is there for the dad and more earthy 14-year-olds, rather like those appalling rhythmic girls who practise dancing each week on Top of the Pops. Of course the return of the Daleks is all Dr Who needs; what the Top of the Pops girls need is something else, but that is neither here nor there."
More recent reviews have also not been positive. Paul Cornell, Martin Day, and Keith Toppingwrote of the serial in The Discontinuity Guide (1995), "An ambitious project which has the look of a grand folly due to budget constraints and the tongue-in-cheek script... K9 makes a quite impressive debut, though, as with many aspects of The Invisible Enemy, the ideas are better than the realisation." In The Television Companion (1998), David J. Howe and Stephen James Walker called it one of the "weakest" Fourth Doctor stories, mostly consisting of "clichéd and undemanding action-adventure material". They also noted the inconsistent visual effects. In 2010, Mark Braxton of Radio Times was not harsh, although he felt there was a "precarious juxtaposition" between good and bad effects, and criticised some incompetent action scenes.DVD Talk's John Sinnott disliked the way K-9 was used too conveniently and found the plot too similar to but less well done than Fantastic Voyage. He priased the visual effects of the inside of the Doctor's head, but criticised the other sets.
A novelisation of this serial, written by Terrance Dicks, was published by Target Books in March 1979.
The story was released on VHS in September 2002. The DVD was released on 16 June 2008 with the spin off "K-9 and Company" in a double pack called "K-9 Tales". This serial is scheduled to be released as part of the Doctor Who DVD Files in Issue 133 on 5 February 2014.