The Xyz were a species with which the Fifth Doctor was familiar.
The Xyz were tall and humanoid. They were various shades of purple. They had trunk-like legs and vestigial arms. To Nyssa, they looked like the kind of alien that would be created by someone "with no knowledge of real alien life forms".
The Xyz were known for dominating wherever they spread, though no one knew how they managed it, as they had no military capabilities. Therefore, the Doctor was surprised to find a group of Xyz on Earth, but soon he learned that they used addictive serial drama — in Earth's case, the television programme Surrender, Earthlings! — to enslave populations. The Doctor and Nyssa defeated this group, who then left Earth before the humans could harm them. (PROSE: Not So Much a Programme, More a Way of Life)