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...