Implementation Units, also known as Imps, were fighting units on an Earth colony visited by the Doctor.
Imps were constructed from biological material and were programmed genetically rather than mechanically.
During the Doctor's first visit to the colony, the Imps helped to fight off an invasion of the planet. When the Fifth Doctor visited the colony again, the Imps were out of control, rampaging through the city. (PROSE: The Comet's Tail)
Behind the scenes
- Because The Comet's Tail seems to be a dream of the Doctor's, it is not clear whether the Imps were real or just part of the dream.