Parts: 6
Writers: Robert Sloman, Barry Letts
Airdates: 19 May 1973 - 23 June 1973
Production Code: TTT
Megalomaniac of the Week: BOSS
Creature Feature: Giant Maggots
Special Guests: UNIT (the Brigadier, Benton, Yates)
Story: The Doctor decides to take advantage of his recently-regained abilities to time-travel, and announces to Jo that they are going to take a trip to Metebelis 3. Jo, however, has other plans - she has been reading about the environmental pollution being caused by Global Chemicals in South Wales, and has determined to join with a group known as Wholeweal to protest.
As it turns out, the Brigadier has business with Global Chemicals as well. A local miner recently contracted a strange ailment while below ground, and died shortly after returning to the surface, his skin glowing a bright green. While the Doctor jets off to Metebelis 3, the Brigadier and Jo head to Llanfairfach - Jo to meet with the fabled Professor Cliff Jones and the Brigadier to talk to Stevens, the president of Global Chemicals.
When the Doctor returns from the blue planet, less relaxed than he had expected, he joins the Brigadier in Wales. He and Jo discover first-hand that frightening things are happening in Llanfairfach - the abandoned mine has become infested with toxic slime and giant maggots that, if they were to get loose, could wipe out the population of Britain, if not the world. But that is not the only threat to the world - if it is not destroyed by giant maggots, the controller of Global Chemicals has plans to dominate it...