Mankind is colonising 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 it over 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 Fourth Doctor, but he is unaffected. 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 K9, 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 K9 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 realises 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 K9 proceed to Titan Base in the TARDIS. They 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 jams 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 K9, who has ably assisted them. Prof. Marius offers K9 to the Doctor, as he is due to return to Earth, and the Doctor and Leela leave with their new companion in the TARDIS.
Production
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 K9 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 K9; the crack was originally concealed, but the scene was reshot with little time left to repair the join.
Cast notes
Michael Sheard makes his fourth of six appearances in Doctor Who, having made previous appearances in The Ark, The Mind of Evil and Pyramids of Mars. Brian Grellis previously played Sheprah in Revenge of the Cybermen and would later appear as the Megaphone Man in Snakedance. Frederick Jaeger also played Jano in The Savages and Professor Sorenson in Planet of Evil.
Broadcast and reception
The story was repeated on BBC1 on consecutive Thursdays from 13 July – 3 August 1978, achieving ratings of 4.9, 5.5, 5.1 and 6.8 million viewers respectively. 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 in the Saturday early evening slot, it was now losing the ratings war. More recent reviews have also not been positive. Paul Cornell, Martin Day, and Keith Topping wrote of the serial in The Discontinuity Guide, "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, 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 K9 was used too conveniently and found the plot too similar to but less well done than Fantastic Voyage. He praised the visual effects of the inside of the Doctor's head, but criticised the other sets.
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 "K9 Tales". This serial was scheduled to be released as part of the Doctor Who DVD Files in Issue 133 on 5 February 2014.
