Around the year 5080, a cloudlike organism - a virus - floats in space between Jupiter and Saturn, waiting for humankind to encounter it. When a ship comes into contact with the organism, lightning-like energy makes contact with the ship, transferring the virus to it. The virus thrives on intellectual activity, and will immediately try to find a host that is showing the greatest amount of mental activity. It transfers itself to its host through another bolt of energy, and then settles into the mind-brain interface. The virus is noetic (only detectable during consciousness) and it attacks the mind and the intelligence.
Those taken over by the virus develop gradual symptomatic characteristics. Bushy, wiry white hair sprouts on the eyebrows, and the skin around the eyes turns scaly and iridescent. More advanced symptoms include appearance of the bushy white hair also on the hands and eyebrows, and spreading of the scaly patches of skin. These signs appear slowly, and disappear if the virus' control is overcome. Infection by the virus also allows the hosts to communicate telepathically with the Nucleus, and they seem to possess a telepathic awareness of other people. At first they can be stunned by radiation, but they gradually develop a resistance to it.
As the virus, which calls itself the Swarm, begins to establish a hold on a group of people, they start working toward the Purpose: to establish a Hive, the ideal place for the Nucleus to breed and multiply. The virus could also be spread by a bolt of energy between the eyes of an infected host and the eyes of a potential host. Those who are not suitable hosts for the virus, because they do not have sufficient intellectual activity (operating, for example, more on instinct and intuition), are considered to be rejects which must be destroyed.
When the TARDIS passes through the cloud, its mental activity caused the viral cloud to transfer the Nucleus to it, and from there it passed into the Doctor's mind. While he was able to maintain control most of the time, the Nucleus could also exert its power from within the Doctor's brain to influence his actions. The humans controlled by the virus were charged with guarding the Nucleus and its host.
The Doctor is taken to the Bi-Al Foundation to diagnose his illness ("an unidentified viral-type infection with noetic characteristics at present seated in the mind-brain interface and therefore having no ascertainable mass or structure"). The Nucleus appears to be a lump of black rock in the Doctor's mind, with one dark eye and a waving white claw. When it escapes through the Doctor's tear duct, and is enlarged by the TARDIS' dimensional stabilizer, it looks like a large prawn with four stubby arms and lots of waving antennae.
The Nucleus plans to return to Titan, where it will breed a new Swarm. Once they hatch, in the macro world, the Swarm will be mighty, invulnerable, and invincible. The Nucleus believes that the Swarm have as much right to conquer humans as the humans have to strike out amongst the stars. It hopes that through the Doctor, they will be able to conquer time as well as space.
It is unclear exactly how the Nucleus breeds, but it somehow manages to lay thousands of eggs. These eggs must be incubated in breeding tanks with precise temperature and humidity, and a methane atmosphere. Once they hatch, they apparently will feed on humans.
The Doctor finds that Leela's resistance to the virus may not be merely a result of her intellect; in her bloodstream is a fishhook-shaped antibody which he is able to isolate, analyze and duplicate. Scientists at the Bi-Al Foundation breed the most powerful strain (C-531) and when this is injected into those infected with the virus, the infection is destroyed. Unfortunately, when the Doctor and Leela arrive on Titan with the antibody, the Doctor loses it, so he is forced to improvise. He rigs a blaster to fire when the door to the breeding tank is opened, and turns on a nearby oxygen tank. When the Nucleus tries to open the door to release its hatchlings, the combined methane and oxygen are set aflame by the blaster fire, causing Titan Base to explode and taking the Swarm with it.
