The one where the Doctor laughs menacingly.
The Doctor and Jo visit Stangmoor Prison, where a new machine is to be demonstrated by Professor Emil Keller. The Keller Machine will apparently extract all negative thoughts and impulses from even the most hardened criminals, but when a prisoner collapses while undergoing the treatment, the Doctor's scepticism looks justified.
Meanwhile, the Brigadier is organising security at a World Peace Conference, where documents go missing and the Chinese delegate is murdered.
Elsewhere, UNIT Captain Yates is escorting a newly-banned "Thunderbolt" missile across Britain to be destroyed.
The Doctor goes to the peace conference and helps the Brigadier foil another assassination, this time of the American delegate. The culprit is found to be Chinese Captain Chin Lee, but she has been hypnotised by the Master — who is currently posing as Professor Emil Keller...
The Master uses the evil impulses stored in his machine (which is actually just a case for an alien parasite) to cause unrest at Stangmoor and then convinces the convicts to hijack the Thunderbolt missile, which he plans to aim at the peace conference and start World War III.
However, the first convict who collapsed, Barnham, is now immune to the machine's effects and shields the Doctor as he takes the machine to the airfield where the missile is being held. Using the machine to keep the Master occupied, the Doctor buys time for the Brig to reconnect the missile's self-destruct mechanism and destroy it and the parasite-machine together. In the confusion, the Master escapes...
This story, despite being filmed in colour, for a long time existed mostly in black and white, as the original colour prints had been lost. For the DVD release in 2013, the second to sixth episodes were recoloured using the automated chroma dot colour recovery method, while the first, whose films did not have chroma dots, was recolourised manually with the help of Stuart Humphryes, more commonly known as Babelcolour.
- Action Girl: Jo does some running round with a pistol - a legacy of her original concept as an Emma Peel Expy, as opposed to a ditzy Ms. Fanservice in platform boots.
- All There in the Script: Some of the character names appeared in scripts but were not used on screen: The Governor's full name is Victor Camford (the television version merely calls him "Victor") and the two warders in the condemned cell are called Johnson and Samuels.
- Badass in a Nice Suit: The Master wears a business suit here instead of the Blofeld-ian Nehru jackets he'd be known for later.
- Badass Boast: The Master faces down the evil after it defeated the Doctor:"You can't harm me. I'm stronger than you are. I brought you here. I gave you the minds you need to feed on! You are my servant! YOU'RE MY SERVANT!! ... I'm too strong for you! ... *once outside, he bars the door* No more minds for you to feed on. Let's see how long starvation takes to bring you to heel."
- You can see him being affected by it, then resisting. In the end, he isn't able to defeat it on his own with sheer willpower, but every time it looks like it will overpower him, he is able to get back to his feet. He is able to shut down the machinery it was attached to, and, when that doesn't stop it, leave the room and bar the door. While it's a rare moment we see the Master in a great deal of distress, again, this thing was able to put the Doctor out of action in minutes, and leave anyone else dead in seconds. The Master is able to fight back, tell this thing exactly who it's dealing with, and then back up every word.
- Bait-and-Switch Gunshot: The cliffhanger for episode 5. Mailer has the Doctor at gunpoint, there's a close-up of a gun firing, end of episode. The beginning of the next episode reveals that it was the Brigadier firing on Mailer to save the Doctor.
- Brief Accent Imitation: The Brigadier puts on a Cockney accent while trying to sneak into the prison.
- Butt-Monkey: Sergeant Benton is this for this story.
- Call-Back: The Doctor's worst fear is the world bursting into flames, as seen in "Inferno".
- The Cavalry: The Brigadier and UNIT stage their Storming the Castle at the right time to save the Doctor from a sticky predicament.
- Chair Reveal: When the Doctor meets the Master in the governor's office.
- Comically Missing the Point: On hearing the Chinese delegate's name, the Doctor remarks that he must be Hokkien. The Brigadier says that no, he's Chinese.
- Continuity Cavalcade: When the Doctor is subject to the Keller Machine, he sees images of his old enemies: Daleks, Cybermen, Ice Warriors, War Machines, Koquillion, Slaar, Silurians, Zarbi...
- Continuity Nod:
- Creator Cameo: The director, Timothy Combe, appears several times as a criminal mook during the action sequences when UNIT storms the prison. This is due to the Troubled Production that required an unplanned extra half day of shooting and no actors were available.
- Delivery Guy Infiltration: The Brigadier infiltrates the prison this way to save the Doctor and Jo from the rioters.
- Does This Remind You of Anything?: The opening scene in which Barnham is suddenly and violently removed from his cell for his sentence to be carried out was intentionally done to make the audience think he was about to be hanged as the procedure for the recently abolished death penalty was pretty much identical. It's made clearer by a comment in the chamber:Kettering: Science has abolished the hangman's noose and substituted this infallible method.
The Doctor: [as Kettering continues talking] People who talk about infallibility are usually on very shaky grounds.
- Emotion Eater: The alien entity disguised as the Keller Machine.
- Empty Quiver: The Master steals the Thunderbolt missile, planning to use it to destroy the peace conference and trigger World War III.
- Eviler Than Thou: The machine tries to attack the Master by using his greatest fear against him (The Doctor laughing at him). Cue the Master putting it back in its place.
- The Farmer and the Viper: As usual, the Master is a terrible ingrate.
- Faux Flame: The Doctor finds himself surrounded by illusory fire due to his exposure to the Keller Machine. However, this Faux Flame could potentially kill him because Your Mind Makes It Real.
- Good Smoking, Evil Smoking: The Master smokes cigars evilly in several scenes.
- Internal Homage: When Benton is telepathically attacked by Chin Lee, the shot of him collapsing against some railings is a perfect copy of Masters' death in "Doctor Who and the Silurians", which had the same director.
- Kick the Dog: The Master running over Barnham after Barnham saved him from the Machine.
- Latex Perfection: The Master uses a rubber mask to achieve the same phone repair guy disguise as he used in the previous story, "Terror of the Autons". It's less "perfection" than before as this time it actually is Roger Delgado in a rubber mask rather than a different actor.
- Left the Background Music On: In the car ride to the prison the Master holds a small radio while an ominous tune plays. He switches it off, it's gone. That's right; The Master has his own portable evil music. It's rather awesome.
- It's "The Devil's Triangle" by King Crimson. Make of that what you will.
- London Gangster: Harry Mailer gives off this vibe.
- Look Behind You: The subversion. Mailer and Vosper have the Doctor and Jo at gunpoint when the Keller Machine appears behind them. Jo tries to warn them, and Mailer assumes it's just a trick.
- Manchild: Barnham after his encounter with the Keller Machine.
- The Master
- Match Cut: After the Master's contest of wills with the Keller Machine, which occurs while the Doctor is still recovering from his own ordeal, there's a scene in the Doctor's cell that ends with a match cut from the Doctor, resting from his encounter with the Machine, to the Master, resting from his encounter with the Machine.
- Mind Rape: The Master does this to the Doctor with the Keller Machine by strapping him into a chair, putting a 'telepathic amplifier' in his ear and forcing the Doctor to experience his own darkest Phobias at the hand of the Machine (apparently monsters and fire). The torture is cut far shorter when the Machine begins affecting everyone in the vicinity and the Master pulls the plug, but the Master points out to the Doctor that it was so bad that one of his hearts had 'stopped completely'. The Doctor is barely functional for several scenes afterwards.
- New Powers as the Plot Demands: About halfway through the story, the Keller Machine spontaneously develops an ability to teleport itself around, to avoid the plot running aground the fact that everybody now knows it is dangerous and to keep a safe distance from it.
- Obscured Special Effects: A hallucination of a woman transforming into a dragon is only shown in an extreme closeup of her face, due to the head turning out to be the only part of the dragon costume that came out looking decent.
- Ooh, Me Accent's Slipping: Tommy Duggan tries, but Senator Alcott's accent comes off as rather weak.
- Prison Episode: A significant portion of the plot takes place in Stangmoor prison, loosely based on the real life HMP Manchester/Strangeways.
- Prison Riot: The result of a feedback loop with the Keller Machine.
- Recut: On the DVD special release, all episodes have been recolourised. There is also a new set of credits for the restoration teams that imitates the credits style of the Pertwee era with the Keller Machine in the background, with each credit warbling in and out like the effect the titular machine demonstrated.
- Redemption Equals Death: A more long-term play on the trope: The prisoner Barnham, who starts out a hardened criminal, is the first to be subject to the Keller Machine, which extracts all the negative impulses from his brain and leaves him a more benevolent if simple-minded person, who goes on to befriend the Doctor and Jo. At the very end, however, he is run over by the Master as the latter makes his escape.
- Sarcasm-Blind: Benton takes the Brigadier's suggestion that he's too delicate and should lie down after reporting that he fainted seriously.
- Secret Path: The prison, being an old fort, has a secret tunnel. Lampshaded by Major Cosgrove, who remarks that an attack through a secret tunnel is like something out of a movie.
- Shout-Out: The Keller Machine was inspired by A Clockwork Orange.
- Simpleton Voice: Barnham sounds like he's a bit slow.
- Sinister Shades: The Master's chauffeur.
- Storming the Castle: UNIT mounts a raid on the prison. Also a literal example, given that a real castle doubled for a prison.
- Stupid Good: Barnham, since he's had all his negative impulses extracted. When he sees the Master has been incapacitated by the Keller Machine, he doesn't know any better than to stop and help him.
- Title Drop: In the last episode, the Doctor is explaining why Barnham can disable the Keller Machine.The Doctor: It's the mind of evil, Jo! I should have realised!
- Trojan Horse: The spearhead of the UNIT forces get into the prison in a van purporting to be delivering food and beverages, which the prisoners let in because they're trying to avoid tipping off the outside world that the prison has been taken over (and also because the van driver emphasises that the supplies include a quantity of booze).
- Ungrateful Bastard:
- Jo saves the Doctor from being mindraped to death, but instead of thanking her, he tells her off for not waiting like he told her to.
- And, later, the Brigadier saves the Doctor from being shot. Instead of thanking him, the Doctor complains about him not arriving sooner.
- And at the end the Master kills Barnham after being rescued from the Machine by him.
- Vulnerable Convoy: The missile convoy that is hijacked by the Master's men.
- The Watson: Several characters share the task of eliciting exposition from the Doctor. Jo, of course, usually does it in scenes she shares with him. After the Doctor and the Brigadier uncover the Master's meddling at the peace conference, Fu Peng complains that he doesn't understand what's going on and insists that the Doctor give a clear explanation. When the Doctor and Master are working together to contain the Keller Machine, Jo can't ask what the Doctor's plan is because she's being held hostage in another room, so the Master has to do it, even though he's a genius Time Lord and ought to be able to follow the Doctor's thought processes without needing to ask such elementary questions.
- "What Do They Fear?" Episode: The mind parasite feeds on negative emotions and can summon up mental projections of a person's greatest fear that are capable of scaring them to death. The Doctor's greatest fear, as mentioned above, is his memory of a world on fire. The Master's greatest fear? A huge, towering vision of the Doctor, looming over him and laughing maniacally.
- Working the Same Case: In the first episode, the Doctor is investigating the Keller Machine, the Brigadier is dealing with disruptions at the peace conference, and Yates is overseeing the missile convoy. All three turn out to be part of the Master's plan.
- World War III: The Master's plan. It's ultimately stopped, luckily.
- The X of Y
- Yellowface: Averted. The story that goes out of its way to use real Chinese actors for the Chinese characters, something that was seen as peculiar at the time. The director, Timothy Combe, hated this trope, though more because it looked unrealistic than for any more politically correct reason.
- Yellow Peril: The American delegate is terrified of the Chinese, which the Master is planning to exploit, while Chin Lee initially seems terrifying... but she's a brainwashed victim of the Master, and the Doctor gets on pretty well with the Chinese delegate.