The Powerful Enemy

Sometime in the 25th century, the Doctor and his friends land on the planet Dido and happen across a spaceship (from Earth) that has crash-landed. Its two occupants - a man named Bennett and a young girl called Vicki - are living in fear of a creature called Koquillion, a native of Dido possibly responsible for murdering other members from the spaceship. However, the Doctor realises after a while that Koquillion is in fact Bennett in disguise; it was Bennett who killed the others so as to conceal an earlier murder that he had performed on ship. In a confrontation with Dido humanoid inhabitants Bennett dies as he falls from a cliff. Not wishing the girl Vicki to be alone, the Doctor takes her with him in the TARDIS as a new companion.