The Doctor and his companion Sarah battle against some of the most heinous foes to emerge from the outer universe: The Pescatons. The Doctor finds himself in the capital city of London, where the population is bewildered and trembling beneath the violent onslaught of a merciless invader.

Who or what is the mighty Zor, whose green slanting luminous eyes glare out from the dark of night like giant emeralds? What is the powerful alien force that is bringing Earth's civilisation to a standstill, threatening to annihilate everything in its path?

This is the story of a dying Planet, of a Deadly Weed, and the merciless Creatures themselves. It is a Challenge to the Doctor—a frightening race against time...