K is for... Kastrons

When a group of scientists from the planet Kastron stole a Metamorphic Symbiosis Regenerator from the Time Lords of Gallifrey they used it to experiment on themselves. They wanted the machine to enable them to gain eternal life. Instead, the generator caused a perpetual mutation. The authorities on Kastron were horrified by their actions and had them exiled on a huge spaceship. This was programmed to come into contact with a planet every 50 years, when one of their number could use a transmat capsule to visit the world in search of a cure for their condition. The Kastrons remained in suspended animation the rest of the time. They had long ago grown bored with the entertainments which their captors had left on the ship.

The scientists had quickly come to realise that their immortality was a curse rather than a blessing, and they now longed for death.

When the spacecraft arrived in orbit around Earth the scientist who travelled down to the planet discovered that there was a Time Lord present. This was Mawdryn. He knew that the regenerative powers of a Time Lord could be tapped to allow them to die. The Doctor would have to sacrifice these powers, meaning that he could no longer regenerate. 

The mutation had become contagious and both Tegan and Nyssa found themselves infected. Mawdryn made use of this situation to force the Doctor to co-operate with them.

As it was, the Doctor did not have to sacrifice his remaining regenerations. Two versions of the Brigadier were present on the ship, from different time zones. When they met each other they shorted out the time differential, and the temporal energy allowed Mawdryn and the other Kastrons to die.

Played by: David Collings (Mawdryn), Peter Walmsley, Brian Darnley. Appearances: Mawdryn Undead (1983).

• The aliens are only referred to as "Mutants" in the cast list. The name 'Kastrons' originates from the novelisation of the story, by its original writer Peter Grimwade.