called Bert to help another man, Dai Evans, who has called for help at the bottom of the mine. When the Doctor and the Brigadier arrive, Dave, the man controlling the cage's descent, finds that the brake has been sabotaged. The Doctor slows the cage's descent, but his efforts leave Jo and Bert trapped at the bottom of the shaft. There, they find Dai, who is turning bright green and dying. Bert remembers there is an emergency shaft out of the mine, and he and Jo set off. The Doctor suggests cutting the mineshaft cables and linking it up to a donkey engine, which would enable him to use the second cage to get down the mine. The Brigadier goes to Global Chemicals to request some cutting equipment, but a man named Fell informs him that they do not have such equipment. The Doctor has Professor Jones and his environmentalist friends create a demonstration at the Global Chemicals gate, while he slips in, in an attempt to steal the equipment. However, he is captured. Fortunately, Dave and the Brigadier, while on their way to get cutting equipment, stopped at a petrol station and found a man using the required equipment to cut up an old car. They borrow this, free the secondary mine cage, and the Doctor goes down the shaft with Dave and two other miners. They find Dai now dead and a note from Jo telling them that she and Bert have headed for the emergency shaft. Jo and Bert have made good progress through the old mine tunnels, when they find a green slime trickling down the wall. When Bert touches this, he begins to grow weak, and his hand starts to turn bright green. At Bert's insistence, Jo goes on without him. Dave and the Doctor find Bert, and the Doctor goes on to find Jo. By the time they reunite, Jo has found a vast lake of bright green slime, filled with huge maggot creatures. When the tunnel collapses behind them, they use an old mining wagon to get across the green lake. They then climb a steep shaft, where the Doctor collects a huge egg to take back for experimentation. At the top of the natural shaft, they find a large pipe, with the insides covered with traces of crude oil waste—meaning that the pipe leads to the Global Chemicals plant. In the plant,