The Visitation, Part One

Attempting to return Tegan to Heathrow Airport in 1981, the Doctor and his companions arrive at the wrong time but the right location. It is London, 1666, and the city is in the grip of the final widespread outbreak of the bubonic plague in British history. The TARDIS crew are not the only "foreign" visitors. Escaped criminals belonging to a race known as the Terileptils are hatching a villainous plot and the future of humanity itself is at risk.