Eldrad was once hailed as the saviour of the world of Kastria. He erected force-barriers around the planet, preventing its imminent destruction. However, when the Kastrians refused to submit to his dictatorship, he removed his force-fields, condemning an entire civilisation to a slow and agonising death. For this, the mightiest of crimes, the people of Kastria sentenced Eldrad to obliteration.
Millions of years later, the Doctor's TARDIS materialises in an English quarry. A freak accident traps the Doctor's young friend, Sarah Jane Smith, under tons of rock. When she is rescued, she is frantically clinging to a large stone hand. Taken to a local hospital, it soon becomes clear that Sarah has been possessed.
After she goes missing - still clutching the hand - the Doctor follows the trail of death and destruction to a nearby atomic power station.
What strange power does the hand have over Sarah? Why has she fought her way to the very core of the nuclear reactor? How can she possibly survive the normally lethal radiation? To answer these questions, the Doctor must travel across the galaxy to a long-dead world - after which, life will never be quite the same again...