Ghost Light

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

The Doctor brings Ace to Gabriel Chase, an old house that she once burnt down in her home town of Perivale. The year is 1883 and the house is presided over by Josiah Samuel Smith, who turns out to be the evolved form of an alien brought to Earth in a stone spaceship that is now in the basement. Others present include the explorer Redvers Fenn-Cooper, who has been driven mad by what he has seen there, and Nimrod, Smith's Neanderthal manservant.

Smith intends to use Fenn-Cooper's unwitting help in a plot to kill Queen Victoria and restore the British Empire to its former glory. His plans are hampered by Control, a female alien whose life-cycle is in balance with his own. Ace inadvertently causes the release of the spaceship's true owner - a powerful alien being known as Light.

Light originally came to Earth to compile a catalogue of its species but, on discovering that his catalogue has now been made obsolete by evolution, he decides to destroy all life on the planet. He disintegrates when the Doctor convinces him that evolution is irresistible and that he himself is constantly changing.

Control has meanwhile evolved into a lady and Smith has reverted to an earlier, primitive form. They leave in the spaceship, along with Nimrod and Fenn-Cooper, heading for new adventures.

13 out of 105 (12.4%) raters say this story requires a previous story.