The Doctor and UNIT are investigating a murder at Project Inferno, an attempt to drill through the Earth’s crust to access the energy at the core. The drilling is producing a green ooze that transforms human beings into Primords—savage creatures than can only be killed by extreme cold. Professor Stahlman (Olaf Pooley) is infected. The Doctor tries an experiment on the TARDIS console, but he is transported sideways in time to a parallel continuum.
In this universe, Great Britain is a fascist republic and the Doctor is captured by an evil alternate Brigadier Lethbridge-Stewart (Nicholas Courtney) of the Republican Security Forces and an alternate Liz Shaw (Caroline John), who is a military officer. When the drill penetrates the crust, it unleashes heat and poisonous gases, and the ooze transforms most of the staff into Primords. The Doctor realizes that the core energy will destroy the entire planet and convinces the team into returning him to his own dimension to prevent the catastrophe. They restore power to this TARDIS console, while fighting off savage Primord attacks, but the alternate Liz kills the alternate Brigadier. The Doctor barely escapes the infernal volcanic eruption.
Back home. The Doctor tries to stop the drilling, but is ignored as always, but Stahlman transforms fully into a Primord and the Doctor kills him, which stops the drilling in time to save the planet. The TARDIS vanishes and ends up in a garbage dump.
The story was inspired by the real Project Mohole, and the evil alternate dimension by 1984--the Big-Brother-like photo on the wall was that of the head of the BBC Special Effects Department. One can’t help wondering if Star Trek’s Mirror Universe had not had some influence, as well. Caroline John enjoyed playing a baddie, but her pregnancy, which had caused problems during the previous story, forced her to quit the show. Nicholas Courtney made a surprisingly good villain, minus the moustache, dressed in Nazi-like garb, and sporting a black eye-patch. His facial expression remained the same, but instead of endearingly uptight, he seemed cold and cruel. Much in the alternate reality reminded me of the series The Prisoner. The Doctor put his Venusian Aikido moves to good use. Inferno was well-received and was later voted the best Third-Doctor story.