Two alien races -- the Jex and the Canavitchi -- are engaged in a battle to invade and either conquer or destroy the planet Earth. The Doctor is summoned to a meeting with Brigadier who shows him a photograph of a powerful media mogul named Sanger -- who has bought enough plutonium to destroy the world ten times over. UNIT are on the case and it seems that Sanger is one of a frightening number of alien Jex who came to Earth twenty years ago to sow the seeds of their ruthless world-domination... When the Canavitchi launch their counter-attack and their alien fleets embark upon full-scale destruction, the Doctor finds himself engaged in a race against time to save planet Earth.