U.N.I.T. is providing the security for a top secret drilling project, and the Doctor is observing. Professor Stahlman hopes to be able to drill into the Earth's crust and release a new energy source which he has named after himself: Stahlman's Gas.
Professor Stahlman dismisses the concerns of Executive Director Sir Keith Gold and exceeds saftey margins in order to speed up the drilling. The drill head starts to leak a green oily liquid, and anyone who touches it is turned into a primeval creature that craves heat.
The Doctor has been experimenting with parallel universes and using the TARDIS is transported to a parallel universe. He finds a similar world, but finds that Earth is ruled by a dictatorship. He finds that the same experiment is going on, but it is at a slightly more advanced stage.
The Doctor is able to find out what is likely to happen if the crust is broken, and there is no way to save this new world from destruction. The alter-egos, learning that there is a way to escape, try to get the Doctor to take them through to his universe. The Doctor manages to evade them and return to his universe, and stop the drilling before any major damage is done.
- Brigadier Lethbridge-Stewart: Nicholas Courtney
- Liz Shaw: Caroline John
- Sergeant Benton: John Levene
- Professor Eric Stahlman: Olaf Pooley
- Petra Williams: Sheila Dunn
- Greg Sutton: Derek Newark
- John Bromley: Ian Fairbairn (1-4,7)
- Sir Keith Gold: Christopher Benjamin (1-5,7)
- Private Wyatt: Derek Ware (1-3)
- Private Latimer: David Simeon (1,3)
- Harry Slocum: Walter Randall (1-2)
- RSF Sentry: Roy Scammell (4)
- Primords: Dave Carter (5-6), Pat Gorman (5,6), Walter Henry (6), Philip Ryan (5-6), Peter Thompson (5-6)
- Patterson: Keith James (6)
- Loudspeaker Voice: Ian Fairbairn
- Radio Voice: Jon Pertwee (5 - Australian version only)
- Technicians: Keith Ashley, Barry Ashton, Alistair Baine, Robert Birmingham, Valerie Bland, Norton Clarke, Alan Clements, Richard Cooper, Michael Earl, Marcelle Elliot, Ronald Gough, Bertie Green, June Grey, Joan Harsant, Derek Hunt, Colin James, Richard King, Sheila Knight, Richard Lawrence, Natalia Lindley, Patricia Matthews, Keith Norrish, Sue Patterson, Judith Pollard, Corinne Skinner, Harry Tierney, Steve Tierney, Cy Town
- Soldiers: Marc Boyle, Alan Chuntz, Les Conrad, Ian Elliott, Royston Farrell, Billy Horrigan, Roy Street, Terry Walsh
|BBC 1||272. Episode One||Saturday, May 9, 1970 5:15 PM - 5:40 PM||5.7M||61%|
|BBC 1||273. Episode Two||Saturday, May 16, 1970 5:15 PM - 5:40 PM||5.9M||61%|
|BBC 1||274. Episode Three||Saturday, May 23, 1970 5:15 PM - 5:40 PM||4.8M||60%|
|BBC 1||275. Episode Four||Saturday, May 30, 1970 5:15 PM - 5:40 PM||6.0M||60%|
|BBC 1||276. Episode Five||Saturday, June 6, 1970 5:15 PM - 5:40 PM||5.4M|
|BBC 1||277. Episode Six||Saturday, June 13, 1970 5:25 PM - 5:50 PM||5.7M||58%|
|BBC 1||278. Episode Seven||Saturday, June 20, 1970 5:15 PM - 5:40 PM||5.5M||60%|
- Ealing Television Film Studios
- Television Centre Studio 3
- Television Centre Studio 6
- Berry Wiggins & Co, Kingsnorth-on-the-Medway, Kent
People who are turned into a monsters by the ooze are supposed to be red-hot (i.e. when one dies lying against a wall, the paint is blistered and burnt) yet their clothes remain undamaged and do not burst into flames!
Episode 1 : In the TARDIS laboratory, on the Nuclear Power Output gauge in the TARDIS laboratory Megavolts has been misspelt as MEGGA VOLTS.
The Mo-Hole Project
The story title, writer credit and episode number blur in over footage of volcanoes and flowing lava.
For episode five only, Caroline John is credited as Section Leader Elizabeth Shaw. This is also the last story to feature the character of Liz, who disappears between seasons.
This is also the last story to have no specially composed incidental music.
Douglas Camfield fell ill during production, so Barry Letts took over as director for some of the story.