It turns out that the locals had been killed off by one of the occupants of the crashed space-craft, and this person then was impersonating the locals. After some strange plot twists everybody was dead except for the remaining crashed-space-craft-occupant, Vicki, who then agreed to join the TARDIS crew.
Maybe they needed a fourth to continue playing bridge?
Seriously, the departure of Susan in the previous episode left a vacuum in the roster. This new character, Vicki, was able to fill the same kind of role which Susan had previously done. Namely the TARDIS crew is two couples, Ian+Barbara and the Doctor+Susan and now Doctor+Vicki. Except the stories don't explicitly lay out just what the relationships are. Were Ian and Barbara a 'couple' or were they simply two people who were riding along on the journeys of the TARDIS?