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The Invisible Enemy

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. 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 paneled 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 favor of adopting a replacement based on the old console design.


Episode 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, realizes 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 behavior. 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.

Episode 2

The Doctor resists the voice in his head, manages to warn 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 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 theater. 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 stabilizer to shrink the clones to microscopic size so Marius can inject them into the Doctor’s body.

Episode 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 miniaturized 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!

Episode 4

The Nucleus explains to the now virus-free Doctor that, having used the dimensional stabilizer 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 stabilizer 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 refueling tanks to blow. After the Doctor nearly leaves without Leela or K0, 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.


The Fourth Doctor