While hovering in space around the year 5000 AD, the TARDIS enters a cloud of energy and is struck by lightning-like tendrils. The TARDIS has become infiltrated by the Nucleus of The Swarm and as it spreads throughout the time machine’s systems it is able to enter The Doctor’s mind where he realises The Nucleus is a space-borne intelligence which wishes to spread itself across the universe.
At first The Doctor does not seem to have been affected by the attack. And so on hearing a distress call The Doctor, along with Leela, decide to investigate. The distress call originates from Titan, one of the moons of Saturn, where, unbeknown by The Doctor, the human occupants of a refuelling station have also been taken over by The Nucleus.
The TARDIS materialises on Titan and as they start to search for whoever sent the distress signal and why. However, it is not long before The Doctor realises that he has become infected. The Doctor also realises that he has been chosen to be the host of the Nucleus of The Swarm, due to his incredible powers as a Time Lord.
|Outside the TARDIS
As a result of the virus inside his body, The Doctor is eventually overcome and collapses. Leela it seems is unable to be infected and so The Nucleus declares her a reject and orders that she be killed. Luckily The Doctor manages to temporarily break free of his infection and as they re-enter the TARDIS he is able to relay to Leela the coordinates of a local hospital asteroid, the Bi-Al Foundation, before he collapses again. Accompanying them is Lowe who, unknown to The Doctor and Leela has also been infected by The Nucleus.
On arrival at the medical station, based on asteroid K4067, they meet Professor Marius who introduces the group to K9, his dog-shaped robot computer. Professor Marius then examines The Doctor and pronounces that there is little he can do. However, The Doctor devises a plan whereby if he clones himself and Leela and then miniaturises the clones, using the relative dimensional stabiliser from the TARDIS, they can then be injected by Professor Marius into his body, in the hope that they can find and destroy the Virus Nucleus. Once their task is completed the miniaturised clones, of The Doctor and Leela can then escape from The Doctor’s body through a tear duct.
Professor Marius agrees, even though the clones will only live for ten minutes due to problems with the cloning technique used, and so he injects the miniaturised clones, of The Doctor and Leela, into The Doctor’s neck. The miniaturised clones then spiral down into The Doctor’s bloodstream. In the meantime, while the sedated Doctor is in a medical room, Leela and K9 fight off the staff members of the hospital who have become infected by Lowe.
Inside The Doctor’s body the miniaturised clones, of The Doctor and Leela have a hazardous voyage through his body. But when they enter his mind they become separated. The Doctor’s clone however, eventually reaches The Nucleus. But he has no weapons with which to destroy The Nucleus. Worse still The Nucleus learns of the intended escape route out of The Doctor’s body as The Doctor thought of it. And so before The Doctor’s clone can stop it The Nucleus heads for The Doctor’s eyes.
And so when Professor Marius collects from The Doctor’s tear duct, what he believes to be the miniaturised clones of The Doctor and Leela and places them into the cloning booth, it is realised that The Doctor’s plan has backfired. As he operates the controls of the relative dimensional stabiliser to return them to normal size, he only becomes aware of the mistake when instead of having retrieved the miniaturised clones of The Doctor and Leela it is the prawn-like Nucleus that appears in the booth. Now enlarged to human size the creature arranges for itself, and the infected medical staff, to be taken back to Titan where breeding tanks have been prepared prior to its invasion of the galaxy.
|Inside The Doctor
With The Nucleus no longer in his body and with the remains of Leela’s clone, and so her immunity factor, introduced into his blood stream The Doctor realises he has been cured. With the infection still spreading throughout the Bi-Al Foundation The Doctor replicates an antidote and gives it to Professor Marius to use on the remaining medical staff.
The Doctor, Leela, and K9 then proceed back to the base on Titan in the TARDIS. There they just barely manage to fight off the infected humans. The Doctor though is able, with the help of K9, to set up a booby-trap by rigging up a blaster behind a door to fire into a cloud of Oxygen gas which is escaping from a tank. As intended, when The Nucleus finally forces open the door, the blaster fires, igniting the Oxygen in Titan’s methane atmosphere destroying The Nucleus and the base.
On returning to the Bi-Al Foundation Professor Marius gives K9 to The Doctor as a parting gift as the Professor has to return to Earth. The Doctor is at first reluctant, but Leela is delighted. In the end it is K9 who decides for himself, between staying with his creator or to accompany the time travellers, when he trundles through the open police box doors. Shortly after The Doctor and Leela follow him in the TARDIS dematerialises, leaving Professor Marius to reflect: ‘I only hope he’s TARDIS trained’.