The Doctor is making adjustments to the TARDIS for a visit to Metebelis Three, when Jo reads about the mysterious death of a miner in South Wales. He was found at the bottom of a mine shaft, glowing green. Jo wants to meet the local Nobel-winning environmentalist Professor Clifford Jones (Stewart Bevan). While the Brigadier investigates the miner’s death, the Doctor insists on finally visiting Metebelis Three. The Brigadier is suspicious of the Global Chemicals Oil Plant, near the mine. The Boss, Stevens (Jerome Willis), claims the Stevens process produces 25% more fuel from crude, but only a minimum of waste. Prof. Jones doesn’t believe it and suspects Global is responsible for the miner’s death. Jo sets out to help him by going into the mineshaft.
The Doctor discovers that Metebelis Three is not the paradise he was told. He is attacked by various creatures and returns to UNIT with only a small blue crystal. He drives to South Wales in Bessie and meets the Brigadier. They go down into the mine and Stevens sends his henchmen after them. Jo has already arrived at the mine and gone with a miner named Bert (Roy Evans) to help another miner Dai Evans (Mostyn Evans) who has called for help. When the Doctor and the Brigadier arrive, Dave (Talfryn Thomas) finds the brakes on the elevator cage have been sabotaged. Jo and Bert find Dai, who is green and dying. Bert and Jo escape up a little-known emergency shaft.
The Doctor suggests cutting the mineshaft cables and attaching them to a donkey engine. The Brigadier goes to Global Chemicals to request some necessary equipment, but a man named Fell (John Rolfe) says they have no such equipment. The Doctor has Prof. Jones and his environmentalists demonstrate at the entrance, while he tries to steal said equipment, but he is caught and shown that the shed is empty. Dave and the Brigadier find a man at a petrol station using the very equipment they need. They borrow it, fix the mine-cage, and the Doctor goes down with Dave to find Dai dead and a note from Jo about their escape-route. Jo and Bert have found green slime on the walls. Bert unfortunately touches this and begins to turn green. Jo goes on without him, the Doctor finds him and Jo, who has come upon a huge green lake. The Doctor finds an egg and takes it with him.
They find crude oil waste being poured into the shaft. Some of the mine workers dump waste on them, but Jo and the Doctor escape. Fell is given headphones by Stevens and is told to kill himself, which he does. That night, the egg the Doctor found hatches out a giant maggot which heads for Jo but kills Hinks (Ben Howard) with green slime. The next morning, the Brigadier detonates the whole mine. The Doctor is furious, and this does not destroy the maggots, who escape into the countryside.
At Global Chemicals, Mike Yates (Richard Franklin) is working undercover, and the Doctor disguises himself to infiltrate, learning that Stevens takes his orders from the top floor of the complex. A supercomputer named BOSS is behind it all. Jo, to please Professor Jones, searches for maggots in a slagheap. Ah, Love! Meantime, Jones has discovered that Jo inadvertently found a cure for the Green Death. He races to save her from the slugs, and they are both threatened by RAF bombing. With the Doctor, they discover that Jo’s fungus is deadly to the maggots, who are about to turn into giant insects. The Doctor uses his blue crystal from Metebelis Three to take down the BOSS computer. At the local tavern, Jo and Professor Jones announce they are to be married. The Doctor gives Jo his blessing and the blue crystal, as a gift, and he quietly departs in Bessie.
Katy Manning appeared as Jo Grant again 37 years later, in the spin-off Sarah Jane’s Adventures—ironic since Elizabeth Sladen, as Sarah Jane Smith, replaced her in the TARDIS. Many companions’ departures were given short shrift, but this one was treated with respect. The Doctor was clearly heartbroken, as the First Doctor had been with the departure of his grand-daughter Susan Foreman (Carole Anne Ford). Katy Manning as Jo Grant was, in my opinion, the perfect Companion—curious, laughing, feisty and impulsive, fearlessly devoted to the Doctor, and then there were moments, particularly when faced with the kind of evil a show about All of Space and All of Time can touch upon, that her mobile face and huge eyes revealed such depth of sadness that it was enough to break your heart. Frankly, I suspect that part of this was her well-known myopia--on the set, the entire crew was constantly guarding her from bumping into walls and tripping over cables. Actresses from Greta Garbo to Dame Judy Dench have used this little trick. I think Mark Harmon does too.