The Brigadier and Jo go to South Wales, where a man inspecting a nearby disused mine has mysteriously turned green ad died. Professor Clifford Jones suspects that hazardous waste from Global Chemical's new refining process is behind the death, and Jo rashly decides to investigate. Another glowing man is dying in the shaft as she and a miner named Bert descend and are trapped, victims of sabotage. The Doctor and Jo have seen giant maggots, which surface after the Brigadier blows up the mine. As Cliff is infected by the green death, Mike Yates infiltrates Global Chemicals where another menace is brewing. Global Chemicals staff are under control by BOSS, a computer who will let nothing get in the way of Global Chemical efficiency. The Doctor must kill the maggots, cure Cliff, free Mike from BOSS,  and say goodbye to a longtime companion.