The first 'mad scientist' of the Pertwee era, Professor Stahlman (Olaf Pooley, in a fine performance of edgy dedication and building spitefulness) is determined to see his 'Inferno' project reach the very end of its task in penetrating the Earth's core for mystery gas, even if it means destroying the world, and turning himself into a deadly savage Primord, in the process.
I personally liked the Primord creatures of Inferno, a welcome horror addition to the already tense and doom-laden atmosphere of Don Houghton's all-time classic seven-parter of 1970.