Doctor Who/Recap/S10/E05 The Green Death

Stevens, listen to me. You've seen where this efficiency of yours leads. Wholesale pollution of the countryside. Devilish creatures spawned by the filthy by-products of your technology. Men... men walking around like brainless vegetables. Death. Disease. Destruction.

—The Doctor gets all eco-warrior

A Welsh miner is found dead, his skin glowing bright green. UNIT is naturally called in to the village of Llanfairfach to investigate. A local environmental group, led by Professor Clifford Jones, has been investigating the work of nearby Global Chemicals and, with the Doctor and Jo's help, they discover that the mine workings are full of giant maggots and green goo, both by-products of Global Chemicals' plant.

The plant is run by Stevens, who has been taken over by the intelligent computer BOSS (Bimorphic Organisational Systems Supervisor), which plans to seize power by linking to every other major computer in the world.

The Doctor uses a large blue crystal he picked up on holiday on Metebelis 3 to break BOSS' hold on Stevens, and Stevens programs BOSS to self-destruct. Meanwhile, the maggots, which are beginning to pupate into giant flies, are destroyed with a fungus.

In the midst of all this, Jo has fallen in love with Clifford Jones, and decides to leave the Doctor to stay with him. The Doctor gives her the blue crystal as an early wedding gift, and leaves, sadly.

Tropes

A.I. Is a Crapshoot: An interesting take on this in that the computer has actually been programmed by Stevens to be a megalomaniac, as BOSS noticed that humans often achieved their goals better through being illogical.
- BOSS was programmed with the objectives of maintaining productive order and ensuring profit for Global Chemicals. It just interpreted those objectives in an extremely broad and amoral manner.
Chekhov's Gun: There are two: the sapphire from Metebelis 3, and the hybrid fungus that Clifford Jones was raising as a meat substitute.
- The dried spores of the fungus cures Clifford Jones of his own infection from the Green Death later on, while clumps of the fungus kill the maggots.
Corrupt Corporate Executive: Stevens.
Disguised in Drag: Third Doctor actually does this... as a cleaning lady.

The Doctor: (having finally met up, in disguise, with Mike Yates) If you say one word...
Mike Yates: I like your handbag.

- Well, The Doctor had to think on his feet and outsmart the security guards, who was tipped off by BOSS.
Disney Villain Death: Poor Fell.
Erudite Stoner: Most of the people at Cliff's ~~commune~~ research facility, but especially Nancy.
Green Aesop
"I Know You're in There Somewhere" Fight: The page quote is part of one.
Immune to Bullets/Nigh Invulnerability: The maggots... bullets, bombs, and even insecticide don't even phase them.
Janitor Impersonation Infiltration: The Doctor disguises himself as a cleaning woman.
Lampshade Hanging: By this point, the Doctor is less than impressed by the revelation that the real villain is a crazy computer.
Land of My Fathers and Their Sheep, boyo
Logic Bomb: Subverted. The Doctor tries to use the Liar Paradox on BOSS, but BOSS completely fails to blow up, stop working, or devote all its circuits to solving the problem.
Master Computer: BOSS is one of these.
Mood Whiplash: It ends with everyone congratulating Jo and Cliff, including the Doctor... who then quietly leaves the party and drives off on his own.
Names to Run Away From Really Fast: Don't name your intelligent supercomputer "BOSS"; it'll give it ideas.
Not Himself: Those under the control of the BOSS.
Not Listening to Me, Are You?: Jo offers Cliff a cup of tea... coffee... arsenic...
Not That Kind of Doctor: Professor Jones.
Redemption Equals Death: Stevens.
Rousseau Was Right: Professor Jones honestly thinks that they can ask Global Chemicals to let them borrow cutting equipment to fix the Llanfairfach mine elevator. The same Global Chemicals he and the rest of the denizens of Wholeweal have been protesting and verbally chastising every other day.
Sickly Green Glow: Every victim of the "Green Death".
Strawman Political: Played both straight and averted in Global Chemicals. While Stevens is a megalomaniac who is just fine with completely destroying the environment and unleashing a biological horror upon Wales if it'll raise Global Chemicals' stock price a tuppence a share, and BOSS is worse, most of people under Stevens, including the VPs, are decent hardworking people who want to do the right thing and basically have to be brainwashed by BOSS to go along with the program.