The search for the fourth segment of the Key to Time brings the Doctor and Romana back to Earth in the present day.
In a small village in Norfolk, former astronaut Lady Millicent Ferril has established an observatory, tracking a meteorite from the Cronquist System. It is a meteorite that almost killed her years before - and perhaps left her not entirely human.
As Ferril's power grows, so does her influence. She can control metal. And anything metal - from a suit of armour to a bicycle - is now lethal.