Somewhere in the United Kingdom, a young woman called Edith Stevens is attacked by her cat, Boris. Elsewhere in the country, Professor Lloyd informs the Fifth Doctor of the epidemic of similar cat attacks that are sweeping the nation. The time traveller then begins to question Lloyd about the current origin of Britain's milk. The Time Lord is surprised to discover that the tanks in which the milk is being stored comes from Houston, Texas. Somehow, the fungus has spread from outer space to the Houston-made milk vats to the cats of Britain.