The Rescue

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:
Arriving on the planet Dido in the late 25th Century, the time travellers come upon a crashed spaceship from Earth. Its two occupants - a paralysed man named Bennett and a young girl, Vicki - are living in fear of a creature called Koquillion, a native whose people have apparently killed the other members of the human expedition.

However, the Doctor quickly deduces that Koquillion is in fact Bennett in disguise; it was he who killed the others in order to conceal an earlier murder he had committed on the ship.

Confronted by two of the humanoid Dido natives - whom he thought he had completely wiped out - Bennett falls from a high rock ledge to his death. As Vicki's father was amongst the murdered crewmen and she is now an orphan, the Doctor offers her a place aboard the TARDIS.

18 out of 134 (13.4%) raters say this story requires a previous story.