The Doctor tries to take Tegan back to Heathrow in 1981. Although the location is right, the year is in fact 1666 - the time of the Great Plague.
The Doctor and his companions meet an out of work actor, Richard Mace, who tells the Doctor of a recently fallen comet. The Doctor begins to suspect alien intervention, and his suspicions are confirmed when he is confronted by an Android who is terrorfying the locals by impersonating Death.
The Doctor finds a group of Terileptils who have crashed on Earth. They plan to depopulate Earth by releasing a plague, using the black rat as a carrier. The Doctor and his companions follow the Terileptils to London where a struggle sets fire to a bale of hay. The Terileptils are destroyed, and the Doctor throws the remaing vials of black plague into the furnace.
The Doctor's companions wonder why the Doctor does not attempt to stop the fire, but he suggests it should be allowed to run its course.
