A freak accident in a quarry leaves the unconscious Sarah clutching an enormous stone Hand.

The Hand is the only surviving remnant of Eldrad, an alien super-being expelled from his planet, Kastria — and it has the power to control the human mind. Using Sarah as its instrument, the Hand goes in search of the atomic energy it needs to regenerate Eldra's body.

Eldrad is determined to return to Kastria and punish his enemies. The Doctor and Sarah are caught up in the terrifying conclusion of a drama of betrayal and revenge that began millions of years ago.