9 May 1970
Nicholas Courtney (Brigadier Lethbridge-Stewart/Brigade Leader), Olaf Pooley (Professor Stahlman / Director Stahlman), Christopher Benjamin (Sir Keith Gold) [1-5, 7], Derek Newark (Greg Sutton), Sheila Dunn (Petra Williams/Dr. Petra Williams), John Levene (Sergeant Benton/Platoon Under Leader Benton), David Simeon (Private Latimer) [1, 3], Derek Ware (Private Wyatt) [1-3], Walter Randall (Harry Slocum) [1-2], Ian Fairbairn (Bromley) [1-5, 7], Roy Scammell (RSF Sentry) , Keith James (Patterson) , Dave Carter [5-6], Pat Gorman [5-6], Walter Henry , Philip Ryan [5-6], Peter Thompson [5-6] (Primords).
|Written by||Don Houghton|
|Directed by||Douglas Camfield/Barry Letts|
|Produced by||Barry Letts|
20th Century Earth. An unhinged scientist, Professor Stahlman, is attempting the first penetration of the Earth’s crust in a top secret drilling project called Inferno. His purpose? To tap into a new energy source at the core. But at what cost?
When The Doctor is called in with his companion Liz Shaw to oversee the project, he soon develops grave misgivings. Things begin to go very wrong when a mysterious green substance leaks from the drillhead. A substance which turns all who come into contact with it into alien primeval creatures called Primords.
Meanwhile, The Doctor finds himself transported into a parallel universe identical to 20th Century Earth.
The mystery deepens as he finds that although the place, time and people are all the same, no-one seems to know who he is.
Only one thing is certain: the drilling must be stopped before the full force of the energy from the core is unleashed, destroying not only this Earth but the one The Doctor has just left behind
- The Master, the Brigadier and Ian return to this timeline in The Face of the Enemy.
- The memory of the Third Doctor later realises just who the Dictator of this timeline was in Timewyrm: Revelation.
- Not counting occasions where The Doctor arrives in a world where history in our universe has been altered, The Doctor visits other alternate universes in The Quantum Archangel, Blood Heat, The Infinity Race, The Domino Effect, Reckless Engineering, and The Last Resort.
- This story had the working titles of the Mo-Hole Project, Operation: Mole-Bore, The Mole-Bore and Project Inferno.
- The TARDIS console is removable and can travel by itself, although it needs an external power source to do this.
- This is the only season finale in the Third Doctor era to not be written by Robert Sloman.
- Olaf Pooley initially refused to put on the Primord costume, but eventually Barry Letts persuaded him to do it and Pooley was later glad that he did agree to wear it.