Doctor Who: The Rescue (S02E011)

Date: Tue Jun 06 2017 Doctor Who
The Doctor an Companions, in need of a new young lady to be the Doctor's friend, land on the planet Dido. It seems the Doctor has been to Dido before and expected to find friendly and peaceful people. Except what they find instead is a crash-landed space-craft and a local who is anything but friendly or peaceful.

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?