N is for... Nucleus of the Swarm

An aggressive virus - the Swarm - existed in deep space, close to the orbit of the planet Saturn. Its sentient Nucleus sought to invade and conquer the macro-universe as well as the microscopic one. The virus was noetic - being spread via mental activity. Anyone entering the region of space was infected through their ship's computer systems. A shuttle crew were converted and compelled to establish a facility on Saturn's moon Titan, in which the Nucleus could create a hatchery. A mayday from the base's commander brought the TARDIS into the area, and the virus was drawn to its telepathic field. Through it, it infected the Doctor - seeing him as an ideal host for the Nucleus.

He sought help at the Bi-Al medical facility on a nearby asteroid, placing himself in a self-imposed coma to slow the influence of the virus, though those already infected fought to abduct him and bring him to Titan.

To combat the infection, the Doctor had himself and Leela cloned then miniaturised, to inject into his body. The Nucleus was found lodged in his brain - appearing as a black mound with a single eye and clawed appendage. They failed to destroy it, and it was able to evacuate his body through the clones' escape route - the tear duct. Once the miniaturisation process was reversed, the Nucleus grew to huge size, resembling a crustacean.

It was ready to spawn so insisted on being rushed to Titan where it lay its eggs in specially prepared tanks. The Doctor and others infected could be cured through a vaccine produced from Leela's blood, as she had proved to be immune to its effects - being a creature of instinct rather than intellect.

Whilst the Doctor admitted that the virus had a right to exist on the micro-level, it could not invade the macro-universe, and so engineered an explosion which destroyed the base seconds before the Swarm could hatch.

Played by: John Scott Martin. Voiced by: John Leeson. Appearances: The Invisible Enemy (1977).

• Leeson was hired to provide two voices for the same story - this and K-9 - by its director. They had worked together before, and bumped into each other in the local pub one night.
• John Scott Martin had by this time become the principal Dalek operator, as well as making a handful of on-screen appearances out of costume - The Daemons, The Green Death, Robot.
• Writers Bob Baker & Dave Martin had described the Nucleus in crustacean terms, but were disappointed in the actual shrimp-like costume, which reportedly kept shedding fibreglass shards over the studio floor.