Doctor Who. The Stones of Blood

"We won't find Dracula hanging around the Manor. But we may find something equally disturbing..."
The Doctor is delighted when his quest for the Key to Time leads him to his favourite planet, Earth. But his friends are less enchanted: Romana is nearly lured to her death by a sinister apparition, and K9 is all but destroyed by a belligerent boulder with the power to move - and a thirst for blood.
An ancient stone circle becomes a battleground as the Doctor must outwit the deadliest alien criminal this side of hyperspace - and her bloodthirsty silicon servants...