The Invisible Enemy ( TV story ) The Invisible Enemy was the second story of season 15 of " Doctor Who " . It introduced the robot dog K9 as a companion to the Fourth Doctor . 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 . Synopsis . 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 . Plot . Part 1 . 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 the 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 disoriented 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 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 . Part 2 . 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 clone s 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 . Part 3 . 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 . Part 4 . The Nucleus explains to the now virus - free Doctor that , having used the dimensional stabiliser to grow , the virus ' s next generation can attack human ity 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 . Home video and audio releases . DVD releases . Contents : Notes : K9 Tales . This story and " K9 and Company " were released as a DVD box set in 2008 . Doctor Who DVD Files . It was also released as issue 133 of " Doctor Who DVD Files " .