The Visitation is the fourth serial of the 19th season of the British science fiction television series Doctor Who. It was first broadcast in four twice-weekly parts on BBC1<!--- name at that time ---> from 15 to 23 February 1982.
The serial is set in and near London in the 17th century. In the serial, a group of fugitive aliens called Terileptils plot to make the Earth their new home by spreading a deadly plague among humanity.
Plot
The TARDIS lands in 17th-century London. Upon stepping outside, the Doctor, Nyssa, Tegan, and Adric immediately smell sulphur and head off to find the source. Richard Mace, a highwayman and self-proclaimed thespian, encounters the group and takes them to safety inside a barn. While questioning Mace, they find out that some kind of comet recently landed nearby.
The Doctor and his friends discover the "comet" was actually a spaceship inhabited by a Terileptil fugitive, who uses mind-controlling bracelets to subdue the local villagers. In a nearby manor house, they find a cellar full of caged rats and a device emitting soliton gas. The Terileptil plan to use rats infected with a genetically enhanced plague to devastate the population and conquer the planet.
Using the TARDIS scanner, the Doctor locates the Terileptil in London. The TARDIS rematerialises there and the five enter the building. With the Terileptil leader are two others, who get the jump on the Doctor and Mace. The Doctor manages to stop them, but the Terileptil leader's weapon starts to overload and detonates. The resulting explosion destroys the building and starts a raging fire. Mace stays behind to fight the blaze as the Doctor, Tegan, Nyssa, and Adric leave in the TARDIS.
It is revealed that the fire is at Pudding Lane, the location where the Great Fire of London started.