Ace and the Seventh Doctor are on the trail of a monster that feeds on fear. It appears to have taken up residence in the subconscious of Sherilyn Harper, the leader of a right-wing nationalist party that's [[NoCelebritiesWereHarmed definitely not the BNP]]. Because these people actually have access to government files, they're perfectly aware that the Doctor worked for the United Nations Intelligence Taskforce in the 70's.
The Doctor has no idea what the monster is or where it came from. He theorizes that it could be a leftover hunting mechanism for a now-extinct species. Whatever it is, it's feeding on the racial tension caused by Harper's political fearmongering, and ''everyone'' -- her opponents, her audience, her commentators -- are feeding that fear. When Harper survives an assassination attempt, the Doctor and Ace get involved in her life and try to get the monster to show itself. The people shooting at Harper, however, turn out to have been hired by... Harper. And they unexpectedly cause people to start rioting in the streets.
The monster turns out to have a few BodySurf tricks up its sleeve, and Ace starts to believe that it's now inside the Doctor. After all, the Seventh Doctor is the scariest damn thing Earth has ever seen. Ace herself is genuinely terrified of him with some frequency, so she can easily imagine the monster feeding on the fear he inspires in people. The Doctor never knew Ace felt that way about him, and he tries to talk her out of a HeroicSacrifice to kill the thing. Because she'd also gladly give her life to save the Doctor. Which he didn't know either. Gently, he explains to her that the monster isn't inside him, but inside ''her'', so Ace drags it out of her subconscious and saves the day.
* ButterflyOfDoom: Discussed in great detail. The Doctor quietly tells Ace that it's "just a story they tell butterflies", and that she should stop looking for the one moment in life that will magically change everything.
* CallBack: Ace tries the Doctor's "look me in the eye, pull the trigger" tactic that he used in [[Recap/DoctorWhoS25E2TheHappinessPatrol "The Happiness Patrol"]]. It doesn't work for Ace, and she gets shot in the shoulder.
* ChekhovsGun: The bomb that Walter plans to blow up Harper shows up at the end of the story, with Ace threatening to use it to blow up the Doctor.
* EmotionEater: The Fearmonger.
* FictionalPoliticalParty: The New Britannia Party. Once again, the NBP are [[FunWithAcronyms definitely not the BNP]].
* FryingPanOfDoom: The Doctor uses one to whack a monster.
* GuileHero: Ace is trying to take a few levels in the trope. It works, briefly, but it backfires on her when she gets too confident.
* MacGyvering: The Doctor fashions a force field out of random stuff in a tool shed.
* RealityEnsues: Ace tries to be a GuileHero, talking down a madman by using the Doctor's tactics. But she's rather inexperienced and, unlike the Doctor, not psychic and not a hypnotist. She gets shot and has to spend three weeks in the hospital.
* ShockJock: Mick Thompson.
* StrawCharacter: Sherilyn Harper and Mick Thompson both qualify.
* TomatoInTheMirror: Who really ''is'' the Fearmonger?
* WeNeedADistraction: The Doctor hopes that climbing on top of a car and ''juggling'' will confuse people into calming down. It doesn't really work.
* WellIntentionedExtremist: Harper. The Doctor tells her that she's ''at least'' doing what she believes in, even if it's rubbish.