Parts: 4
Writer: David Fisher
Airdates: 28 October 1978 - 18 November 1978
Production Code: 5C

Doctor: fourth
Companion: the first Romana, K-9 mark two

Megalomaniac of the Week: Cessair of Diplos
Creature Feature: Ogri

Story: The Doctor and Romana locate the third segment of the Key on Earth - where, under cover of night, a Druidic cult holds a ceremony in a local stone circle. Bearing bowls full of blood, they pour their tribute onto two of the stones, which begin to glow...

The next day, the Doctor and Romana arrive at the circle at Boscombe Moor, where they meet Professor Emilia Rumford and Vivien Fay, who live nearby and study the stones. When the Doctor spots blood on the ground near some of the stones, Miss Fay brushes his concerns aside, certain that it results from the animal sacrifices carried out by the cult. The Doctor decides to pay a visit to the cult's leader, De Vries - but the man has been warned of his coming, and plans to kill him as a sacrifice to his goddess, the Cailleach. Romana, waiting for him at the circle, is relieved to see him return near sundown...but to her horror, the Doctor leads her to the cliffside and then pushes her over the edge.

Someone is determined to keep the Doctor and Romana from finding the third segment - someone who has been on Earth for a very long time indeed. And with them are the Ogri, silicon-based lifeforms that pose as members of the stone circle and have a great thirst for blood...