Tony Adams

In Wales, a dead miner is found glowing green. UNIT investigates as Jo empathises with the environmentalism of Cliff Jones, who is opposing the pollution from the company, Global Chemicals.

The Doctor discovers that Global Chemicals hides a sinister secret. While investigating the old mine workings, Jo and miner Bert become trapped beneath the surface when the only lift is sabotaged. Bert touches some green slime and becomes infected.

The Doctor and Jo discover the mine's terrible secret. Global Chemicals' managing director, Stevens, is determined to make sure that the two of them won't live to reveal what they have learnt.

The Brigadier attempts to seal the giant maggots in the mine with explosives, but this only results in the creatures burrowing to the surface. Meanwhile, the Doctor is trapped by the real boss of Global Chemicals.