R is for... Ransome, Thea

A scientist employed by industrialist Dr Fendelman, who sought to study the human race's earliest origins. During an expedition to central Africa, a skull had been unearthed which analysis showed to be millions of years older than the date currently accepted for the appearance of the human ancestor. 

The skull was brought back to a secluded mansion near the village of Fetchborough, which Fendelman had recently bought to use as a research centre. Along with Thea, he also employed scientists Adam Colby and Max Stael. With the latter, he was conducting secret experiments with a time scanner. He hoped that this would let him see into the distant past, but it was highly unstable.

With the humourless Stael busy with Fendelman, Thea bonded with Adam. 

She began to suffer blackouts, during which she was drawn to the skull. It had a crack in the shape of a pentagram at its crown - a feature which Thea also had.

At one point she collapsed, and several small slug-like creatures appeared briefly on her body. The Doctor recognised these as embryonic Fendahleen which, when fully grown, comprised a gestalt creature with a humanoid core - the Fendahl. 

This creature devoured the life force and if left unchecked would destroy every living thing. The Time Lords thought they had trapped and destroyed it on the fifth planet of the Solar System, blowing up the entire planet and confining the remains within a time loop. However, the core had escaped to prehistoric Earth, wiping out life on Mars on its way there.

The skull from Africa was all that was left of it - and Thea was fated to become the new core.

She was abducted by Stael, who secretly led a local coven devoted to the Fendahl.

Thea was transformed into a golden skinned being who could turn people into Fendahleen.

The Doctor used the time scanner to set up a controlled implosion which destroyed the new Fendahl, then dumped the skull into a supernova to prevent it from ever being resurrected again.

Played by Wanda Ventham. Appearances: Image of the Fendahl (1977)

• Second of three appearances for Ventham in the series. She had previously played Jean Rock in The Faceless Ones, which also featured Donald Pickering. The pair were reunited in 1987 for Time and the Rani.
• She was also considered for the roles of Todd in Kinda, Dr Solow or Lt. Preston in Warriors of the Deep, Jane Hampden in The Awakening, and Queen Katryca in The Mysterious Planet.
• Another notable sci-fi role was as Colonel Virginia Lake in Gerry Anderson's UFO.
• Horror roles include The Blood Beast Terror (1968) and Captain Kronos - Vampire Hunter (1974).
• She is the mother of actor Benedict Cumberbatch, and featured in Sherlock as the consulting detective's mother.