Investigating the old mine workings, Jo and Bert become trapped beneath the surface.
The Doctor reaches Llanfairfach, with a blue crystal to show for his visit to Metebelis III, and jams the lift in the nick of time. Jo and Bert are not harmed, but they are stranded in the mine. There, they find Dai Evans, one of the other miners, glowing bright green and terribly ill. It seems there is some serious pollution at work in the mine and they head off to find a way out. For anybody to get down to the mine to help them, cutting equipment is needed to cut the cables jamming the lift. Global Chemicals say they don’t have any such equipment, but a staff member, Elgin, knows this is not true. When Jones hears about Jo’s predicament, he comes to the mine to help.
The Doctor sneaks into Global Chemicals to steal the equipment but he is caught. Fortunately, Stevens chooses to overlook the incident. By chance, the Brigadier finds somebody with cutting equipment in a garage, allowing the Doctor and a few mine workers to go down into the mine shaft. Things get worse when Bert finds a slick of green slime and touches it. He too seems to contract “the green death”. By the time the Doctor finds Dai, the miner is dead, and the Time Lord becomes very worried for Jo’s safety. Increasingly ill, Bert waits behind while Jo looks for a way out alone. The Doctor catches up with Bert, and finds Jo near a vast lake of green slime filled with giant maggots. When they try to go back, the tunnel caves in and more maggots emerge from the rubble.