In the console room of the TARDIS, the Doctor is discussing with Adric the previous adventure of Deva Loka. Meanwhile, Nyssa is helping Tegan pack, as they should be landing at Heathrow Airport soon. Tegan and Nyssa enter the console room to find they have indeed landed at the location of Heathrow, but 300 years early. Tegan storms out. The quartet gathers outside and, smelling sulphur, go in search of the source. They are attacked by Villagers, but escape. Adric loses his Tardis-homing device and the group is separated. A highwayman and sometime actor, the bombastic Richard Mace (Michael Robbins) discovers the lost group and takes them into a barn where he is hiding out.
Mace mentions that a comet has recently landed nearby, a harbinger of the plague. The Doctor knows it was not a comet. He notices the necklace Mace has found, which is actually a bracelet used for prisoner control. They search the barn and find alien power packs, which indicates there were survivors of the so-called comet strike--really the breaking up of a crashing spacecraft. They head off to the nearby Manor House to find the landowner and get some information. Nobody answers the front door, so the Doctor and Nyssa climb in through a window. They find more power-packs, gunpowder, and traces of a high-energy weapon, but no people. The Doctor notices a wall in a strange place, blocking a stairway. Nyssa lets the rest of the team in through the front door and they find the Doctor missing. As they stare at the wall, someone locks the door behind them.
The Doctor appears through the wall and calls it a holographic energy barrier. They walk though and smell soliton gas, find cages of rats and the device emitting the gas. A bejewelled Android (Peter Van Dissel) sneaks up behind them and stuns Tegan and Adric. The rest of them run. The Android’s master is a Terileptil fugitive (Michael Mella), who interrogates Tegan and Adric. The Doctor and the others find the spaceship’s life craft near the manor and decide to take on the alien with a sonic booster from the TARDIS. A group of villagers attack them. They hide in the ship, now under siege. The Doctor blasts open the rear hatch and they escape into the forest, pursued by the villagers. In the Manor, Tegan and Adric are locked in a room. Nyssa heads back to the TARDIS to build the sonic booster, and the Doctor and Mace decide to question the local miller (James Charlton), who seems knowledgeable. Adric escapes the Manor through a window, but Tegan is caught by the Android. Getting nothing from the miller, the Doctor and Mace leave the mill, and are attacked again by the villagers as plague carriers.
The Terileptil sends the village headman (Eric Dodson) to fetch the Doctor and Mace. They are thrown into a room at the mill. At the Manor, the Terileptil puts a control bracelet on Tegan. Adric is helping Nyssa set up the sonic booster. The Doctor disables some of the control bracelets. The Android, in the mask and robes of the Grim Reaper, frightens the villagers, seizes the Doctor and Mace. The Terileptil refuses the Doctor’s offer to take him home in the TARDIS, as he is a wanted fugitive there. He would prefer to kill everyone on Earth and take the planet. He destroys the Doctor’s sonic screwdriver, then reveals that the rats in the cages are infected with a powerful plague.
The Doctor overcomes the mind-controlled Mace and Tegan with the power packs he has picked up. The Terileptil leaves for the city with his rats on a wagon and sends the Android to take control of the TARDIS, but Nyssa uses the sonic booster to destroy it. The time-travellers and the astonished Mace escape and head for London. The Doctor locates the Terileptil and his lieutenants in London. The TARDIS materializes there, a struggle ensues and the Terileptil’s weapon overloads and detonates, starting the Great 1666 Fire of London, which destroys the plague. The TARDIS crew leaves as Mace remains to fight the fire.
The story received favorable reviews as an engaging action romp, though criticized for its awkward complexity. The flamboyant stage-actor Mace, played by Michael Robbins, was engaging and probably the best thing in the story. Peter Davison said the story was one of his three favourites. The sonic screwdriver, like K9, was too tempting a problem-solving plot-device and the producer wanted it taken away from the writers, so the Terileptils broke it. The sonic screwdriver did not return until 2005. The Terileptils had escaped from the Tinclavic Mines on Raaga, where they were serving life for their crimes. The name Terileptil comes from Territorial Reptiles. Normally, in that filming location, a plane would fly overhead every three minutes, which meant that the production was constantly held up and they were running out of time. But an air-traffic controllers’ strike was called, and the planes stopped flying, so they were able to finish on schedule.