Episode three, directed by Michael Briant, written by Robert Sloman, BBC1, tx. 02/06/1973
Reunited with his companion Jo, the Doctor realises that the mine in which they are trapped is infested with giant maggots. At the top of the mineshaft, Professor Jones holds Global Chemicals responsible for the deaths in the mine. Preventing him from going underground, the Brigadier decides to visit Global Chemicals himself.
Taking a giant maggot egg for examination, the Doctor and Jo find a mine cart and punt past the teeming maggots. They begin to climb an access pipe, and from the crude oil waste on its sides deduce that it leads to Global Chemicals.
At the plant, Elgin blames his co-worker Fell for the deaths of two miners, after Global Chemicals' refusal to allow UNIT to borrow cutting equipment. Fell emotionlessly denies all knowledge of what has been happening in the mine, and is followed to a control station by the increasingly suspicious Elgin.
The Brigadier tells Stevens that he is taking charge of the plant, pending a full United Nations investigation into Global Chemicals. Fearing that "sentimental fools" could cause his plant to be shut down, Stevens complains by phone to the Minister of Ecology, who is in a cabinet meeting. The Minister orders the Brigadier to put UNIT at the disposal of Global Chemicals. When the Brigadier protests that it is no longer a domestic matter, the Minister passes the phone to the Prime Minister, who reiterates the order.
The Doctor and Jo are detected in the pipe by Fell, who orders the waste to be flushed into the pipe regardless. Elgin protests, and at the last moment Fell overcomes his mind control and helps the Doctor and Jo escape. He rushes to see Stevens, who forces a set of headphones onto his head. The computer controlling the operation decides that the processing has been unsuccessful, and orders Fell's self-destruction. Fell stands up and walks to the nearest fire escape, throwing himself to his death.
At the Nuthutch, after serving a fungus-based meal, Professor Jones describes his plans to travel up the Amazon to further his experiments by investigating the natives' dietary habits. The Doctor arrives and reports that the third miner had been killed by a virus. Jo is upset at her involvement in the death of the miner, and she and Jones kiss. As he leaves, the rescued egg hatches and a maggot emerges, slowly crawling towards an unsuspecting Jo.
