This story returned the Fourth Doctor and Leela to the original TARDIS console room from the beginning of season 14, now with a considerably updated appearance, with the changes in its features and the fact the Doctor had reverted to using a different console room all this time being explained as a deliberate attempt to redecorate the console. During the production break between the 1976 and 1977 seasons, the newer, gothic-style TARDIS console set that had been designed to improve the comfort of camera crew's filming duties was put into storage. However, the wooden panelled walls of the set proved to be its greatest flaw when they warped out of shape while the prop was not being used. The damage was too extensive to repair, and the new set had to be discarded in favour of adopting a replacement based on the old console design.
A three-man rocket crew are nearly done with their mission to Titan Base until a course change puts the rocket in the path of a strange cloud in space. By the time they arrive, they have come under the control of a sentient virus which threatens the galaxy. When the TARDIS picks up an emergency message, it flies into the cloud, infecting the Doctor. To save himself and others, he must undertake a dangerous journey.
A small spacecraft flies through an asteroid belt. Despite the efforts of the three-man crew, it is drawn into an anomaly. An energy discharge strikes the ship and infects its computer with a virus, which declares, "Contact has been made."
By the time the ship reaches Titan Base, the three crewmen have been infected also. They kill the resident crew and reveal their slowly changing faces. When the station supervisor, Lowe, realises the men he knew are now trying to take over the base, he sends out a distress call.
The Fourth Doctor and Leela, back in the refurbished console room, intercept the distress signal and proceed towards Titan. They pass through the same anomaly. The Doctor is struck by the energy, collapsing on the floor. He eventually recovers but seems disorientated and briefly has trouble speaking. Despite Leela's warrior instincts telling her there is danger and evil on the station, the Doctor insists they answer the mayday. The shuttle crew are already aware of the TARDIS' imminent arrival and prepare for the coming of "the Nucleus". Lowe tries to stop them, killing Silvey, but Safran and Meeker chase him into the cryogenic section and lock him in.
The Doctor and Leela separate and explore the station. Leela finds a frozen Lowe in the cryogenic section and helps him recover, learning of the shuttle crew's strange behaviour. The Doctor meets Safran and Meeker, who use the energy discharge to further infect him. They order him to kill Leela, who is immune and therefore useless to them. When the Doctor finds her, Meeker insists that he is not in danger from the Nucleus that infests the Doctor's body. Leela kills Meeker, but not before he has infected Lowe. As Lowe leaves to follow Safran, the Doctor creeps up behind Leela, his hand covered with silver fur, and aims a weapon at her back.
The Doctor resists the voice in his head, warns Leela and collapses, his infection receding. Lowe and Safran confer and decide that, while Safran prepares the station for incubating the virus, Lowe shall protect the Doctor at all costs. When the Doctor insists on finding medical help, Lowe suggests the Bi-Al Foundation in the nearby asteroid belt. The Doctor gives Leela the co-ordinates before he puts himself in a coma, and she pilots the TARDIS there.
The Doctor is taken to Professor Marius, an expert in alien diseases, who at first dismisses the Doctor as a spacenik. Lowe fakes an eye injury and infects a doctor with the virus. They begin infecting more staff members. Marius becomes much more interested in the Doctor's case when his robotic dog, K9, reveals the Doctor's extraterrestrial origin and that he is infected by a virus residing in the mind-brain interface. The Doctor wakes himself and discusses the virus with Marius and K9, who was built by the professor to replace Kelso, the dog he left on Earth. They decide that the virus thrives on intellectual activity. Since Leela is an instinctive being, it cannot infect her.
Marius prepares to operate on the Doctor's brain, which makes the virus cause a shuttle to crash into the asteroid as a distraction, while the infected staff launch an attack on the operating theatre. Leela and K9 hold off the infected crew while the Doctor forms a new plan, preparing short-lived clones of Leela and himself before lapsing back into his coma. The Doctor's clone uses the TARDIS' dimensional stabiliser to shrink the clones to microscopic size so Marius can inject them into the Doctor's body.
The clones make their way into the Doctor's brain, dodging electrical impulses and the Doctor's immune system. They look for the mind-brain interface, where the Nucleus has hidden itself, and any neural damage that may show the way. As they proceed, they discuss the complexity of the brain and how it operates.
Lowe demands Marius surrender the Doctor, so Leela and K9 destroy the service shaft and barricade the corridor to slow the infected. The barricade helps until one of them infects K9 with the virus. K9 is ordered to kill Leela, but she dodges the blast and is merely stunned, while K9 shuts himself down to reboot. Lowe infects Marius and kills Dr Parsons, but the nurse escapes and finds K9 and Leela in the corridor. Lowe has himself cloned and is miniaturised and injected into the Doctor to stop the other clones.
The cloned Doctor and Leela cross the mind-brain interface and track down the Nucleus. The Doctor tries to convince it to leave, but the Nucleus asserts its right to survive and procreate like any other life form, explaining that it has hung dormant in space for millennia. It compares itself to the human pioneers swarming out into space. Now that it has access to the Doctor's TARDIS, it can spread the virus through time and space. The cloned Lowe is stopped by Leela's duplicate and the Doctor's phagocytes but, just as the Doctor's clone tries to shoot the Nucleus, the time limit runs out and the clones vanish. The Nucleus escapes from the Doctor's body through a tear duct and is enlarged by Marius to human size. The Virus has been brought into the human world.
The Nucleus explains to the now virus-free Doctor that, having used the dimensional stabiliser to grow, the virus's next generation can attack humanity on a macroscopic level. The age of Man has ended. The age of the Virus has begun. The Doctor is not impressed, having heard such boasting before. Leela disguises herself as an infected nurse and frees the Doctor so they can take refuge in the TARDIS with K9. Without the dimensional stabiliser they cannot leave. They watch as Lowe and his infected brethren help the Nucleus onto a shuttle for Titan.
K9 stuns Marius so the Doctor has time to examine his own blood and discover that Leela's clone has left him with antibodies against the virus. He replicates the antibodies and cures Marius, who can replicate the cure for his staff. The Doctor plans to eradicate the virus spawning on Titan, but Leela insists they simply blow it up. When the cure is ready, the Doctor borrows K9 from the Professor and heads for Titan Base.
The Nucleus arrives at the base just in time for spawning and enters the incubation tanks prepared by Safran. When the TARDIS arrives, Leela discovers the infected are developing resistance to her blaster. K9's weaponry is more effective, but his energy levels are dropping quickly. He is able to determine that those aboard the base have been infected for so long that their condition is now irreversible, and that the Doctor's priority should be destroying the Nucleus.
K9 draws the infected away while the Doctor sneaks up on the spawning tanks. Lowe confronts him and makes him lose the antibodies, but K9 uses the last of his power to shoot Lowe, who is absorbed by the swarm. Leela kills Safran with her knife while the Doctor alters his plan and rigs the refuelling tanks to blow. After the Doctor nearly leaves without Leela or K9, the trio escape the base just in time to see the massive explosion, amplified by the methane in the atmosphere, from orbit.
Returning to the Bi-Al foundation to restore K9 to Marius, they find the antibodies have been a success. Marius surprises the Doctor by asking him to adopt K9; weight requirements will not allow the professor to take his robot dog with him when he returns to Earth. K9 happily accepts the situation and departs with the Doctor and Leela, leaving Marius to wonder if the robot is TARDIS trained.
- Doctor Who - Tom Baker
- Leela - Louise Jameson
- Voice of K9 - John Leeson
- Lowe - Michael Sheard
- Safran - Brian Grellis
- Meeker - Edmund Pegge
- Silvey - Jay Neill
- Crewman - Anthony Rowlands
- Nucleus - John Scott Martin
- Voice of Nucleus - John Leeson
- Professor Marius - Frederick Jaeger
- Parsons - Roy Herrick
- Cruikshank - Roderick Smith
- Marius' Nurse - Elizabeth Norman
- Reception Nurse - Nell Curran
- Opthalmologist - Jim McManus
- Hedges - Kenneth Waller
- Medic - Pat Gorman
- Computer voice - Anthony Rowlands (DWM 271)
- Titan Base Crewmen - Harry Fielder, Stuart Myers (DWM 271)
- Bi-Al Members - Leslie Bates, Ken Sedd, Alan Clements, Cy Town, Derek Hunt, Margot Gordon (DWM 271)
- Assistant Floor Manager - Tony Garrick, Christabel Albery
- Costumes - Raymond Hughes
- Designer - Barry Newbery
- Film Cameraman - Nick Allder
- Film Editor - Glenn Hyde
- Incidental Music - Dudley Simpson
- Make-Up - Maureen Winslade
- Producer - Graham Williams
- Production Assistant - Norman Stewart
- Production Unit Manager - John Nathan-Turner
- Script Editor - Robert Holmes
- Special Sounds - Dick Mills
- Studio Lighting - Brian Clemett
- Studio Sound - Michael McCarthy
- Theme Arrangement - Delia Derbyshire
- Title Music - Ron Grainer
- Visual Effects - Tony Harding, Ian Scoones
- The Doctor suggests a "kind of St. Elmo's fire" is responsible for the halo of light that momentarily surrounds him.
Biology and medicine
- K9 states the first successful cloning experiments were carried out in the year 3922. He adds, "The Kilbracken holograph cloning technique replicates from a single cell a short-lived copy. Efficiency of individualisation not completely guaranteed."
- Professor Marius is said to be an expert in extraterrestrial pathological endomorphisms.
- Professor Marius takes an encephalograph out of the Doctor.
- The Doctor says he has lost the ability to tune his brain's Reflex Link with "The Time Lord Intelligentsia - a thousand super-brains in one."
- Leela tells the receptionist that the Doctor is from Gallifrey. The receptionist believes it to be in Ireland.
- This story was listed on the 1970s Doctor Who Sound Effects LP as The Enemy Within.
- The idea of injecting miniaturised people into someone's body in order to solve a medical problem, and especially the exit through the tear duct, are strongly reminiscent of the film Fantastic Voyage and its novelisation by Issac Asimov, of which the writers drew inspiration for this story from .
- This story featured more extensive model work than any previous Doctor Who story. (INFO: The Invisible Enemy)
- According to Matt Irvine, the model used for the cratered surface of Titan was a re-use of the model used for the surface of the Moon in Space: 1999. (DCOM: The Invisible Enemy)
- The story re-introduced the "primary" TARDIS console room, now slightly redesigned by Barry Newbery. It had been abandoned for the wooden, "secondary" control room in the previous year's season opener. It would remain in use, with modifications, until season 25.
- Leela is left-handed, or at least writes with her left hand. Actress Louise Jameson is right-handed, but chose to make Leela a left-handed writer in order to increase her awkwardness at this task.
- In the Titan Base and Bi-Al Foundation sets, all signage is written phonetically in what the script calls "Finglish" — thus, for example, signs read "IMURJINSEE EGSIT" and "ISOLAYSHUN WARD" instead of "Emergency Exit" and "Isolation Ward". (INFO: The Invisible Enemy)
- This is the only story in which the monitor on K9's left side actually displays anything.
- John Leeson is credited as "Nucleus Voice" for part one, while for parts two to four he is billed as "Nucleus & K9 Voice" on-screen and as "Nucleus Voice and K9" in Radio Times.
- The Radio Times programme listing for part two was accompanied by a black-and-white full-length publicity still of the Doctor, Leela and K9, with the accompanying caption "The Doctor and Leela are joined by a new ally, K9, who'll be helping them in the fight against The Invisible Enemy. Dr. Who: 6.5".
- The introduction of K9 Mark I marks the beginning of an almost seven-year span of stories featuring at least one non-human companion which lasted until the departure of Turlough, a native of Trion, and the destruction of Kamelion, a shape-shifting android, in Planet of Fire in 1984.
- John Leeson later reprised his role as the Nucleus of the Swarm in the audio story Revenge of the Swarm in 2014.
- Working titles for this story included [The] Invisible Invader, The Invader Within, and The Enemy Within.
- Tom Baker had terrible trouble acting opposite the Nucleus because he could not take it seriously. At one point, he started singing "Prawn Free". Finally, he blurted out, "How am I supposed to act to a f*cking prawn?"
- Part one - 8.6 million viewers
- Part two - 7.3 million viewers
- Part three - 7.5 million viewers
- Part four - 8.3 million viewers
- Ealing Television Film Studios, Ealing Green, Ealing
- Bray Studios, Slough
- BBC Television Centre (Studio 6), Shepherd's Bush, London
- The first establishing shot of the Bi-Al Foundation shows it with the damage later caused by the shuttle crash.
- When Lowe presses the distress button in his office, the entire wall shakes.
- 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.
- The knife that Leela kills Meeker with is very obviously loosely sewn to his clothes. It wobbles back and forth after he falls.
- When K9 first meets Leela, a reflection of a moving crew member can be seen on the front of the cloning booth.
- In the very first scene of part two, as the Doctor collapses to the ground, someone can be seen moving at the back of the set.
- When K9 shoots one of the infected men, the blast beam appears to come out of his eyes, then moves down to his snout as the camera moves.
- When Marius's nurse meets Leela and K9 in the corridor, the camera and crew members can be seen reflected in her head-mirror.
- When the Doctor is congratulating himself on blowing up the Swarm, the shadow of a boom mike can be seen.
- The Swarm remained dormant in the TARDIS' computer until the Doctor's seventh incarnation. Shortly afterwards, the Doctor returned to Titan Base in the company of Ace and Hector Thomas in 4920. The TARDIS had been commandeered by Hector, who had fallen under the control of the Swarm. It ensured its own creation by returning to the Bi-Al Foundation, where it mutated from the Saturnian plague. (AUDIO: Revenge of the Swarm)
- K9's creation is explained in PROSE: Tautology and One Man and His Dog.
- While clones of the Doctor and Leela are miniaturised in this story, the Doctor would later himself being miniaturised with his companion during his twelfth incarnation and injected into a Dalek. (TV: Into the Dalek)
Home video and audio releases
- PAL -
- Region 4 - 4 September 2008
- PAL -
- Region 1 - 2 September 2008
- NTSC -
- This story was released on DVD on 16 June 2008 in a K9 Tales Box Set. It was released in the box set alongside K9 and Company.
- Early Region 2 versions of the box set feature a fault on The Invisible Enemy disc. A scene from half way through episode 3 is skipped and appears after the closing credits. 2|entertain was aware of the problem but decided to go on with the release as planned. They fixed the problem for later copies of the DVD box set.
- Commentary by Louise Jameson, John Leeson, Bob Baker and Matt Irvine
- Dreams and Fantasy - Making-of documentary
- Studio Sweepings - Raw studio footage from the serial
- Visual Effect - Modelwork in The Invisible Enemy
- Blue Peter - Clip from an episode featuring K9
- CGI Effects - Option to watch serial with some original effects shots replaced by new computer-generated images
- Trailers and Continuity
- Photo Gallery
- Production Subtitles
- Radio Times Listings
- Easter Egg - K9 appears on The Generation Game. To access this hidden feature, press left at 'Visual Effect' on the Special Features menu.
- Editing for the DVD release was completed by the Doctor Who Restoration Team.
Doctor Who DVD Files
- The story is available for streaming in the US through Hulu Plus or Amazon Instant Video in the UK.
- The Invisible Enemy at the BBC's official site
- The Invisible Enemy at RadioTimes
- The Invisible Enemy at BroaDWcast
- The Invisible Enemy at Shannon Sullivan's A Brief History of Time (Travel)