Do you think for once in your life you could manage to arrive before the nick of time?
— The Doctor to the Brigadier
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 Three
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.
- 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.
- Call Back: The Doctor's worst fear is the world bursting into flames, as seen in "Inferno".
- This becomes Harsher in Hindsight after all the mentions of his own home planet 'burning' in the Last Great Time War.
- 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...
- Does Not Understand Sarcasm: Benton takes the Brigadiers suggestion that he should lie down after reporting that he fainted seriously.
- The Master
- 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.
- 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.
- Vulnerable Convoy
- "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. This sets up for a Call Back in "Last of the Time Lords," where The Doctor becomes briefly omnipotent, but instead of laughing maniacally, he does something even more terrifying to The Master — he forgives him.
- World War Three
- The X of Y
- 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, claiming to be a friend of Mao Zedong (which is, in retrospect, a bit odd).
- Then again it might have been a VERY strange scenario like what happen with Nero and the doctor waxed the truth as it were