The Stones of Blood

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Doctor, Romana and K9 are led by the tracer to the Nine Travellers, a circle of standing stones on Boscombe Moor in present-day England, but the third segment is nowhere to be found. They meet elderly archaeologist Professor Emilia Rumford and her assistant Vivien Fay, who are surveying the site, and learn that the circle appears to have had a variable number of stones over the years.

The Doctor encounters a group of Druids led by a man named de Vries and narrowly avoids becoming their latest sacrifice. De Vries is later killed by one of the stones from the circle - an Ogri, a life form that lives on blood. Miss Fay is the latest guise of the Cailleach, a being worshipped by the Druids, who has been on Earth for four thousand years. She transports Romana to a spaceship suspended in hyperspace at the same coordinates as the stone circle.

The Doctor follows and accidentally releases two justice machines called Megara, which sentence him to death for breaking the seal on their compartment. When they attempt to carry out their sentence, however, he tricks them into knocking Miss Fay unconscious. Reading her mind, they learn that she is really Cessair of Diplos - the alien criminal they were originally sent to try. Having established her guilt, they transform her into an additional stone in the circle, but not before the Doctor has grabbed her necklace - the Seal of Diplos, alias the third segment of the Key to Time.

52 out of 82 (63.4%) raters say this story requires a previous story.