Parts: 7
Writer: Don Houghton
Airdates: 9 May 1970 - 20 June 1970
Production Code: DDD
Creature Feature: Primords
Special Guests: UNIT (the Brigadier, Benton)
Story: In need of more energy to power his experiments in restoring the TARDIS, the Doctor joins the team of Professor Stahlman as one of his advisors. Stahlman is doing literally groundbreaking work: using innovative drills, he has been able to penetrate deeper into the planet than ever before. His ultimate goal is to drill through the Earth's crust to access the pockets of gas beneath it, for this gas - named Stahlman's gas in his honor - is a vast and untapped energy source.
But Stahlman has become obsessed with achieving penetration of the crust with as much speed as possible, no matter what the risk. Stahlman ignores all warnings and disregards safety procedures...even going so far as to sabotage equipment in order to keep the dangers secret. But even Stahlman does not comprehend the horrible threat posed by the green sludge which has begun leaking from one of the drilling cores - just one touch turns a human into a regressed primordial beast.
When the Doctor's warnings go unheeded, he retreats to the lab to continue his experiments...but a surge in power sends him hurtling sideways through time. To his horror, the Doctor finds himself in a parallel world where Brigade Leader Lethbridge-Stewart and Section Leader Elizabeth Shaw control the drilling site with an iron fist. Arrested as a spy and under constant threat of imminent execution, the Doctor realizes that the situation on the parallel world is dire indeed. The drilling here is near completion, primordial creatures are stalking the corridors, and the Doctor has very little time to win the trust of his captors. For unless they co-operate with him, the Earth's crust will be breached, unleashing a fiery inferno that will destroy the entire planet...and if the Doctor dies, his Earth is doomed.