P is for... Primords

When human beings came into physical contact with a strange green slime, released from deep within the Earth's crust, a genetic mutation took place. They reverted to a primordial state - hirsute and savage. With some individuals, the process took time. First the skin would begin to turn green and they would experience a crippling ringing sound in their heads. They would crave increasing levels of heat. 

The Doctor and UNIT encountered these creatures at a drilling project - nicknamed "Inferno" - in central England. Professor Stahlman believed that a gas found only deep underground could be exploited as a new source of cheap energy and he refused to heed scientific advice in his pursuit of it. A technician named Slocum was the first to be infected by the slime, and he became homicidal. Anyone attacked who survived became infected and would begin to mutate. It took multiple bullet wounds to kill a Primord, though they were susceptible to cold temperatures.

Stahlman recklessly handled some of the slime and became infected as well, though he was able to keep the mutation in check for a time due to his obsessive nature.

As the animalistic behaviour took over and they mutated fully, victims felt compelled to make others like themselves as well as kill - deliberately exposing them to the slime. This fate befell the Sergeant Benton from an alternative Earth. Here, the professor's project was much more advanced and the Doctor encountered several fully mutated Primords.

They perished along with the planet - destroyed by the natural forces unleashed by the drilling.

The Doctor was able to return to the normal universe where only one person fully mutated - Professor Stahlman. Stunned by CO2 fire extinguishers, he was shot dead by the Brigadier.

Played by: Olaf Pooley (Stahlman), Walter Randall (Slocum), Derek Ware (Private Wyatt), Ian Fairbairn (Bromley), Dave Carter, Pat Gorman, Walter Henry, Philip Ryan, Peter Thompson. 

Appearances: Inferno (1970)

• The Primords are never named on screen. The name was given to them by producer Barry Letts - derived from "primordial".
• The script suggested ape-like creatures - a devolved version of humanity - but director Douglas Camfield opted to make them more like werewolves.
• Olaf Pooley was resistant to wearing the full Primord make-up.
• Walter Randall was one of Camfield's stock repertory of actors. After playing the priest Tonila in The Aztecs he was El Akir in The Crusade, and then Egyptian warrior Hyksos in The Daleks' Master Plan. He was also an IE guard in The Invasion, and finally one of the human overseers in Planet of the Spiders.
• Ian Fairbairn was another of the Camfield rep company - appearing in The Invasion as Gregory and The Seeds of Doom as Dr Chester. He had earlier played Questa in The Macra Terror.
• Dave Carter was, like Pat Gorman, a regular extra / monster performer in the series. He had already played the Old Silurian in Season 7, as well as a plague-infected ambulance driver in the same story.