Surrender, Earthlings! was an addictive serial television programme broadcast on Earth by the Xyz.
The programme featured aliens invading Earth. The aliens were, at least according to Nyssa, unrealistic. The violence of the programme was graphic and convincing.
Both Nyssa and the Fifth Doctor learned that the programme was quite realistic. The aliens depicted were the Xyz, and they murdered the actors on the programme — the realistic violence was in fact real. Humans tended to become addicted to the programme, allowing the Xyz to take over Earth.