The Visitation

From: http://www.bbc.co.uk/doctorwho/classic/episodeguide

The Doctor attempts to take Tegan back to Heathrow Airport but the TARDIS arrives in the 17th Century instead of the 20th. The time travellers discover that a space capsule has crash-landed nearby and that its alien occupants, three Terileptil prison escapees, intend to wipe out all indigenous life on Earth by releasing rats infected with an enhanced strain of the great plague.

The creatures are also using a sophisticated android to strike terror into the local villagers. Aided by itinerant thespian Richard Mace, the Doctor tracks the Terileptils to their base in Pudding Lane, London. The creatures are ultimately destroyed when a fire breaks out and the Terileptil leader's weapon explodes - also setting off the Great Fire of London.

7 out of 84 (8.3%) raters say this story requires a previous story.