Professor Stalman (Olaf Pooley) is heading up something known as Project Inferno,an attempt to drill into the Earth's core for a nearly inexhaustible energy source. Meanwhile many of the people involved are experiencing unusual mutations. UNIT and the doctor are called in to investigate but it's soon apparent the Professor's erratic behavior is part of the same symptom. The concept of drilling to the Earth's core is proving controversial. Same for the doctors plans to use the projects nuclear reactor to restore the functioning of his TARDIS. On his second attempt to do so he finds himself in a despotic parallel universe and a run in with dictatorial alternate version of Lethbridge Stewart and his companion Liz Shaw. They are undergoing the same experiment in this universe. But despite the doctors attempt to tell them he's not a spy,their Earth is destroyed by the successful tapping of the Earth's core and the resulting lava flow. This energy allows the doctor to return to his universe in the TARDIS to warn them on ending the ill conceived project in time.
This is one of the most intense,edgy and even stressful of the original Doctor Who serials. While at first following a similar plot line to other Pertwee era stories dealing with corporate short sightedness,particularly involving nuclear power the real captivating parts of this serial occur during the fourth,fifth and sixth episodes which take place almost solely in a parallel reality. Liz Shaw is portrayed here as a prisoner of her worlds dichotomy,which seems to indicate the up rise of some kind of British version of the Stalin era communist party or even Nazism. Lethbridge Stewart on the other hand is portrayed similarly to his counterpart only more dramatically aggressive and with self centered motivations rather than the generally selfless ones of the Brigadier. Seeing Nicholas Courtney sans moustache and plus eye patch and scars is worth it alone. It's also a reminder perhaps of the dark and light sides of each person's personality,personified here in different universes. One of the strongest and most shocking perhaps of Jon Pertwee era Doctor Who episodes.