I found this to be very similar to the previous "Ambassadors of Death" series with the "Doctor" (Jon Pertwee) and his assistant "Liz" (Caroline John) recruited by the "Brigadier" (Nicholas Courtney) to help solve some mysterious goings-on at another top secret government establishment. This time, it's "Prof. Stahlman" (Olaf Pooley) who is leading a team determined to drill into the Earth's core and release an unlimited source of cheap energy. Of course, this isn't anywhere near as straightforward as it sounds and the deeper they go, the more unusual the side effects on the scientific team who are working on the project. Things become even more complicated when a power surge opens a time tear and the doctor ends up in a parallel universe where a much more militaristic government is in charge of the self same proceedings. This new regime think the "Doctor" is up to no good, so he has to convince them otherwise then manage, somehow, to make it back to his own timeline and save the day. This story is a better constructed one, with the characterisations of the increasingly maniacal "Stahlman" and the comically-scarred alternative "Brigadier" working quite well as the story allows Pertwee to clearly have some fun leading us on a scientific joyride. The special effects are borderline pantomime, but that doesn't really matter as this eight-part drama builds nicely to a tense conclusion. I've heard of death by firing squad, but never by fire extinguisher....