The Doctor vs. alien space lizards, with bonus arson!
Short Synopsis
The Fifth Doctor and his bickering TARDIS crew land in 1666 England, stumble across some hostile alien Terileptils, and stop them from wiping out humanity with a plague-spreading scheme. Unfortunately, in the process, they burn London to the ground. History teachers everywhere still weep.
Extended Storyline
The TARDIS lands in the English countryside during the late 17th century, which means plague, wigs, and suspicious villagers with pitchforks. The Doctor, Tegan, Nyssa, and Adric (yes, he’s still here) quickly run afoul of some locals who assume they’re up to no good. Honestly, given this team’s track record, it’s a fair assumption.
Before the angry mob can skewer them, a highwayman named Richard Mace swoops in. He’s flamboyant, theatrical, and clearly having more fun than everyone else combined. Mace quickly becomes the star of the story, partly because he’s basically playing himself as if he’s wandered in from a different TV show.
The real threat is the Terileptils, reptilian aliens with a taste for Shakespearean villainy and sparkly rainbow prosthetics. They’ve escaped from prison and set up shop in a country manor. Their brilliant plan? Spread plague-infested rats around London to wipe out humanity so they can move in. Because if you’re going to colonize Earth, you might as well make it unhygienic first.
The Doctor tries reasoning with them, but Terileptils aren’t really the “sit down over tea” type. Cue a lot of sneaking through forests, getting captured, escaping again, and Adric nearly getting everyone killed (as per usual). Nyssa, to her credit, gets her moment to shine by whipping up a fancy anti-alien sonic device back in the TARDIS.
The final confrontation takes place in London, where the Doctor and his companions chase the Terileptils into a bakery. Yes, a bakery. During the fight, an energy blast causes the building to catch fire. The Doctor and his team escape, the Terileptils are roasted (literally), and history is changed forever — because that bakery fire becomes the start of the Great Fire of London.
So, technically, the Doctor saved humanity from alien plague rats… by burning down a good chunk of its capital city. Swings and roundabouts, really.