Aired May 19 - June 23, 1973
Written by: Robert Sloman
Directed by: Michael E Briant
Something sinister is happening in the Welsh village of Llanfairfach. One of the miners employed by Global Chemicals has been found dead, his skin glowing bright green. While UNIT is called in to investigate, Jo was already planning a visit after reading about a local environmental research group, led by Professor Clifford Jones, who are protesting the actions of Global Chemicals.
Jo, Cliff and the Doctor discover that the mine is infested with a green slime-like substance and giant ferocious maggots, both of which have been created by the toxic waste manufactured by the company's latest activities. However, the Brigadier's hands are tied as Global's director, Stevens, has convinced the British government that their work will provide cheap renewable energy.
However, Stevens is not truly in charge. Instead, he is working at the direction of the Biomorphic Organizational Systems Supervisor, or BOSS, a sentient super-computer that has integrated human emotions into itself and desires to run the world more efficiently than humanity.
As the maggots begin pupating into giant flying insects, the Doctor discovers that a hybrid fungus of Prof. Jones' invention is capable of destroying them. Still, BOSS plans to seize power by linking itself to every other major computer system across the globe, a plan the Doctor is only able to thwart by using an alien crystal from Metebelis 3 which fully frees Stevens from BOSS' hypnotic control at last. Finally, as BOSS begs his friend for his life, Stevens programs the computer to self-destruct, killing them both.
In the end, Prof. Jones and Jo have fallen in love, and decide to marry. They tell the Doctor they are going to leave England on an Amazonian expedition. The Doctor gives Jo the Metebelis 3 crystal as a wedding present, then slips quietly away from her engagement party, driving off in to the setting sun alone.